[Nexus 9 Q&A, Help & Troubleshooting] TheMadKing:
So... now that I think with hindsight, I did something pretty stupid, and I'm going to need some help, since I have no idea what to do.

I have a Nexus 9 tablet, and I never liked it. The OS is just so slow and laggy, and it overheats, and it has many other problems. Quite a couple of times, I installed custom ROM's and so on, and I never had a problem. Let me explain what I did now.

Around a month ago, while looking on the internet, I heard this thing that if I install the original software, but with no encryption, it would work better. It sounded cool at the time, so I downloaded the Nexus Root Toolkit, and did it.

Everything went well. But then I heard about a custom ROM that was apparently working even better, Dirty Unicorns, so using the same toolkit, I installed it. For the first time, my Nexus 9 actually worked the way it is supposed to work, but the custom ROM still had its problems, it crashed from time to time. I was thinking about possibly returning or selling the tablet, so that's why using the Toolkit, I reinstalled the original software. But, if I went all the steps, and also locked the OEM, my device then showed a flag saying that it was a development device, while booting up. So, after messing around with the settings of the Toolkit a little, I decided to reinstall the software, but leave the bootloader unlocked.

But after some time, I reinstalled my Windows, and I also reinstalled the Toolkit. But that's when I started to have problems. My tablet wouldn't connect to my laptop anymore. I used the toolkit and uninstalled drivers, reinstalling others quite a couple of times, and that's where I think that I messed something up. I managed to eventually connect my tablet, and I rooted it, after rooting it, I connected it again. But adb wasn't working for some reason. A little annoyed, I simply copy pasted the zip file of CM13 into my tablet's internal storage, and I rebooted it to the recovery, TWRP. But the recovery failed to flash it, saying something about an Error 7.

Now that I think about it, I was stupid, since after that, I performed a complete factory reset, resetting my TWRP backup too. I tried to connect it again, nothing. It didn't connect at all. I plug it in, my computer doesn't even recognize that something is plugged in. No Nexus 9, no Android Device, no anything. I booted it in fastboot mode, and it worked, my computer recognized it, and after I downloaded a ROM zip file again, I tried to connect the tablet back and flash it manually, but nothing. It didn't recognize anything anymore.

I tried to reinstall the drivers countless times. I deleted what I had, reinstalled it, nothing. I uninstalled and reinstalled the Toolkit. But strangely, now, even if I know that I installed the Google USB drivers, and the ADB drivers, as I even installed Android SDK, if I use USB Deview, it doesn't show me any of them. I can't see any Google or ADB drivers, although I tried installing them multiple times. So now, I'm stuck with a tablet with TWRP and no OS, which I can't connect to my PC, and I can't use ADB or fastboot.

Is it something from my laptop? I can't check to see if it will connect to another PC right now, so I don't know for sure. Is there any way to delete all the traces from my (failed) attempts at reinstalling the drivers, and to install them again properly? Or is it something from the tablet itself? I don't have an OS, so I can't enable USB debugging.

So, shortly, I have no idea what's happening. And I'm quite obviously a noob in all of these. Can anyone help me? Please?