My iPad has died!
None of the buttons make any difference, no sound, no display, plugging into charger makes no difference, connecting to computer doesn't register on iTunes, NOTHING WORKS!!!
If this is a clue to what is wrong, I had left it running a pdf file in Goodreader - I just didn't switch it off after reading it, as I was busy with something else. I assumed it would power off if it needed to.