I have a Kindle Paperwhite ereader that's over three years old. It's worked well, which is just as well as I use it every day. Occasionally, though, it freezes and I have no choice but to reboot the device and then everything is back to normal.
Today, that didn't work. Repeated attempts at pressing the on/off button were in vain.
I was weighing up whether to buy a new one when I decided to check Google, the fount of all knowledge, and found the answer. Amazon said that in such circumstances I needed to press the on/off button for no less than 40 seconds and the Kindle would spring back into life. It did. And now I can read again.