This happened to me once, a few days ago. I don't remember exactly how I solved it, but I know it was one of the usual troubleshooting fixes that got my iDevice updating again.
Try force closing the app:
Double tap the Home button to show the multitasking bar, then slide the image above the app icon towards the top of the screen.
If this doesn't help, reset it:
Press and hold Home and Power button until you see the Apple logo. Let go of both. Your iPad will power up on it's own.
This kind of reset doesn't remove data or settings. It might help fix the issue you're experiencing.