[Windows Phone 8 Q&A, Help & Troubleshooting] Kepavi:
Greetings everyone. First of all, I am nowhere near experienced as some of the members here, so please don't take this guide for granted. I cannot be held responsible for any damage you might cause to your device. Use it on your own risk. Also it will not be techie written, and I will skip the rage and frustration I had before figuring out how to fix it.

Note: This is not a guide how to revive dead/bricked phone! You must have operational phone affected with issues described below.

That being said, let me explain first how this happened to me so that you can have a picture if your case is similar to mine or not.

About a year ago, I have jumped on Insider wagon and decided to install Windows Phone 10 Preview build on my Nokia Lumia 1520. Like any other geek, I have decided to go "hard-core" and went for Fast ring. During that time, I have done several (well all builds that were pushed to the Fast ring insiders), and not so many Phone resets. Basically, I haven't done factory reset after every upgrade. At one point, somewhere after Astoria was removed from WP, since that's when problems started to appear, I have realized that some features are no longer work on my Lumia 1520. Namely, Glance screen stopped working, Hey Cortana kept reporting that my phone don't support that feature, and most importantly my PC stopped detecting that my phone was connected. First two could be due the bad MSFT code, but last one was game breaking for me.

It wasn't the cables, drivers or hardware issuse

Since I have tried solving the PC connection issues by posting on various forums, pretty much everyone had the same "solutions"

Switch cables and use one you got with the phone

Re-install drivers

Try connecting to MOBO USB instead on front panel on the case

You have fried your phone board/USB/whatever hardware issue

I knew that none of that was the issue in my case simply because the damn thing worked just a build ago, and most importantly because the friend with the same phone model had exactly the same issues. Also, when connected to the computer, phone charged as it should, and when restarting the phone I could hear audible sound from the OS on PC, the same you hear when you connect and disconnect any other USB device. Also, each time I connect the phone to PC, it charged correctly, and Windows Phone gave me notification that phone is charging slowly. Indication that USB is not dead as suggested in the replies on other forums (yes, I know that it can charge but not still be broken, but this was not the case)

However no matter what, Windows Device Recovery Tool was unable to even detect the phone, let alone to start recovering.

How did I solve it?

So, if you experience the same issues as I am, you can do the following to try getting your phone back to 8.x (depends on the latest 8 branch release), and get back Glance, Hey Cortana and USB connection to PC.

This is how it was MY course of action (might be different than yours)

Find out exactly which phone product code you have. It can be tricky with non-replaceable battery such as Lumia 1520. Since I know that when I got my phone I had Wind stuff on it, and that default language installed was Italian, I figured out that my device is RM-937 (this info you can get in About phone, as well as Carrier and Manufacturers name) Wind carrier and black case. Point your browser to http://www.lumiafirmware.com/ and find your phone files. This is crucial part since flashing wrong firmware will, as you suspected, render your phone dead. I figured out that my product code is 059V743 and downloaded ALL the files in the latest firmware avail on that site. In my case there were 16 files.

Install Windows Device Recovery Tool from Microsoft site

Navigate in Windows Explorer or whatever file manager you are using on your PC to C:\ProgramData\Microsoft\Packages\Products\ and there create a folder RM-937 *(thanks to pankaj981 for the location)

Copy all the files from the step one into directory you have just created in step 3. Reason I do this, is because I cannot be sure for how long WDRT will keep connection to my phone (read below how to make it even detect the phone), so I have prepared the files myself. All this assuming that you know what your product code is of course. If not, read on.

Do a phone reset. The reason I do this, is because in that case you will be missing several apps on your phone that are "required" to make WDRT initially detect your phone. Basically, you want to install one of the following apps on clean phone: Gadgets or Glance or any other app that have been re-designed for Windows 10 which require phone restart before first use! I named only two, but there should be more (most of them are listed on WP10 under Settings). If you have issue with finding them on store (heard some people had), you must have them being installed in some previous version of Preview builds. Go to your store app, History, select All apps and find Gadgets or Glance in long list of apps. If you have issues with your history in phone app (heard some people had that issue too), last resort is to install SysApp Pusher app on your device which can be found here. Open it and find Gadgets (might work with other System apps like Extras but not tested) and finally install the application.

Connect your phone to PC. Run WDRT, and watch it struggle detecting your phone. Go to my phone not detected and watch it some more. At one point it will suggest to do Soft reset - Don't.

Open Gadgets app on your phone. The phone will throw you dialog saying that phone must be restarted before using the app. Tap Restart now. Here is where in my case EVERY time phone never restarts itself. It keep showing the dots and if you keep it that way, it will stay in that state forever. Instead, while the phone is still connected (for the sake of security, make sure you are using solid USB cable and will not pop out from the USB slot in the middle of the flashing), do a soft restart by pressing Volume Down and Power button. The phone will vibrate and restart itself. In that moment of complete retardness on your Windows Phone 10 (whatever build you are using), WDRT will detect your phone. You will get something like this

[IMG="WDRT Phone detection"]http://i.imgur.com/YzgfcRW.jpg[/IMG]

This is where WDRT is detected your phone and suggested the image to flash. If you had no idea what your product code is, if you get the screen from WDRT similar to the one above, you should get one file created in C:\ProgramData\Microsoft\Packages\Products\RM-937, in my case called RM937_059V743_02540.00019.15234.50003_049.vpl. Red part is your product code, so head to step 1 and download the rest of the files localy. Or just press Install button and wait for them to download and start flashing. If you previously had files in the named directory, WDRT will validate them, and if all ok starts flashing.

Sit back and wait for process to finish, and when the phone restart, watch your windows detect the phone now correctly, as well as WDRT detect your phone properly. Gratz, if you had luck like I had, you got your phone back to 8.1, and if you are like me after all the frustration, switch off Automatic updates and wait for slightly stable WP10!

Thank you all for reading, if mods think that this is not good enough to be called guide, do whatever you want with it. Otherwise, click Thanks, Subscribe, share it, tweet it, print it, send it to MSFT or do whatever you want with it.

Cheers