That is the original PW3 -
Do not try to use the KT2 firmware on it (as reported in your first post) -
Do not try to use the PW2 firmware on it (as given in the link you posted) -
Only use firmware for the PW3!
And for that model, nothing older than the 5.7.4 PW3 firmware.
Anything older than that version stands a good chance of bricking the device.
The jail break instructions you should be following is at:
https://www.mobileread.com/forums/sh...d.php?t=275877
Do not use any directions similar to those found elsewhere on the 'net.
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Now, tell us more about that: "Will not restart with a long press of the power button".
Describe in the greatest detail you possibly can exactly what you are doing when you try to restart the device with the power button.
How long are you holding the button? (time it do not guess)
What can you observe? Changes in screen display, Changes in the status light, and if doing the re-start with the USB cable connected to a PC, any reaction made by the PC.
Tell us what OS your PC is running.
How are you certain that the USB cable and USB port you are using are working? How did you determine that?
This is a technical forum, and we are being asked to solve a technical problem, we really, really, really need the above information.
Especially since we can not be certain what firmware you tried to install (you have posted two different firmwares - neither for the model of device you have).
It is extremely unlikely to not be restarting.
The long hold does a hardware reset, hardware timer, hardware physical reset line - no software involved.
The only thing that will prevent that from operating is a totally dead battery.
Have you put the device on a wall charger?
Using a known good USB cable?
For how long?
With what external visible changes noticed?
Have you had this Kindle taken apart?
Have you had the case open?
Have you already tried to make a serial port connection?
With what result?
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Translation: So far, your problem description does not make any sense at all.