iPad 2 hang -won' shut down/restart, can't connect to itune

I was sending out a large PDF from iBooks last night and this morning the mail still shows it's sending it. To cancel it, I tried to shut down the iPad. But now it hangs on a blank screen with a progress ring, won't shut down, won't reboot. And when I connect it to iTunes, it says "Cannot connect to this iPhone because an unknown error occurred. Please help!