Well, a reboot seems to be first choice. So it does not do anything when you hold the power switch long enough? Please try to even exceed those 30 seconds.
If that wouldn't work, you could either let it run out of power on its own (which will take days, weeks, months?) or open it and press the hardware reset button (see here:
https://www.mobileread.com/forums/showthread.php?t=72779) or remove the battery for a short moment (I don't know if that would be easy to do...).