The tablet has a lot of really obscure bugs, in case @gabrielg’s suggestion doesn’t turn up any signs of actual hardware failure, all the advice I can offer is to just keep rebooting it, rebooting into recovery, rebooting back into the OS, back into recovery, etc. Another user and I had a problem after the latest update where the tablet told us it was permanently locked after supposedly entering too many wrong pincodes (even though we did no such thing), and it turned out to be yet another massively weird bug that ended up fixing itself after doing about half a thousand reboots holding all sorts of button combinations.