Sounds like a memory problem.
Double press home button to reveal the multitask list. Close everything down by long pressing one app until the minus sign appears. Touch the minus sign on each app to close it. When they've all gone four finger swipe down the screen to close the multitask bar.
If it doesn't work after this, do a reboot. Hold down home and off buttons together until the Apple logo appears then release and wait. After a couple of minutes your iPad will restart and you should be good to go.
Try what followthesapper said first. We were posting at the same time. His suggestion will be quicker.
If these don't work, post back and we'll try some more stuff.
Sent from my iPad 4 HD mini