I had the same problem (as well on a jailbroken iPad). It took me a while to enter DFU mode, but it works and you will be fine. Connect your iPad to your computer, hold power and home button, wait until the Apple logo turns off and on again, release power button but hold Home buttoun until the iTunes and USB symbol appears. iTunes now recognizes your iPad again and offers to restore it - after you have done this you will be fine. Good luck!