Maybe, we should compile a list of steps to try in order of mild to extreme to get the "wont power off" problem fixed.
I am glad I didn't have to go back to very start.
I think from what others have said is that file system corruption is the root cause of the problem.
Not ejecting the device before removing from the computer or the reader encountering a badly formatted/corrupted ebook or the reader doing something weird every now and again when the battery is low could be a few causes.