[One (M8) Q&A, Help & Troubleshooting] redpoint73:
I strongly suggest you reword the title of the thread. Just because

you

don't know what to do, doesn't make the phone "stupid". Coming onto this forum, and name calling (the device) is not a good idea. Most folks who you are asking help from, must hold this device in fairly high regard, to be spending their free time to help others with it.

While well-intentioned, it's apparent your knowledge of this device, as well as some common Android tools, is pretty limited. Therefore, you're making some mistakes and incorrect assumptions. That isn't an insult. You only know what you know, and we all hard to start somewhere.

You can only run the RUU which is intended for the device's specific CID. There are many (dozens?) of RUUs for the different M8 versions. So you aren't going to find the right one by guessing. In fact, I don't think the your M8 version has an RUU (at least, not for free).

Recovering a device without RUU is possible, but it's somewhat involved. It's going to require an investment of time and effort (and some knowledge gained - not altogether a bad thing).

Fastboot works by connecting the phone to a computer, which needs to have a fastboot environment setup on it. Again, I can go into it. But I want to make sure you are willing to put forth the time and effort, to go through with the whole process. Otherwise, advising you on how to use fastboot in itself is not going to get the phone up and running, and not worthwhile.

Can't help or advise on this, unless you are specific about what those guides are, or what specific actions/methods you are attempting. I know you don't have enough post to put up links. But you can at least describe what guides, if you want specific advice on them.

---------- Post added at 01:36 PM ---------- Previous post was at 01:03 PM ----------

Installing TWRP is not related to the OS getting "stuck". TWRP (custom recovery) boots independently of the Android OS, and in fact installing TWRP doesn't even require the OS to be working at all. It just requires being able to access bootloader-fastboot mode (which you can).

But installing TWRP is going to fail, unless you first unlock the bootloader.

You can use the following guide to unlock the bootloader (see Section 1): https://xdaforums.com/htc-one-m8/general/vomerguides-m8-bootldr-unlock-s-off-t2800727

But keep in mind that some files (like TWRP version) in the guide are outdated. I recommend current TWRP version 3.1.1-0, which you can get from here: https://dl.twrp.me/m8/

Only perform the part of the guide relevant to unlocking the bootloader and installing TWRP. No need to root (and the SuperSU file linked is outdated), or any of the rest of the guide.

---------- Post added at 01:42 PM ---------- Previous post was at 01:36 PM ----------

You are on the right track: restore stock TWRP backup, since no RUU available for 6.12.61.4

But I'd suggest just using the original XDA thread, which is referenced in the Android forums thread you linked: https://xdaforums.com/htc-one-m8/help/tutorial-how-to-stock-stock-twrp-t3086860

Just follow up through Step 9, and reboot. The rest isn't needed (no more updates for this phone).

I suppose the Android Forums post has a bit more explanation. But ckpv5 deserves full credit for doing all the work posting the files and maintaining the thread.