Chances are it's running low on memory. Double click the home button, and you'll see a row of apps at the bottom of the screen. Long press one of them until they start to jiggle, then press each one in turn to close them. This will NOT remove them from your iPad.
If you still have problems, try a reboot. This is achieved by pressing both home and standby buttons together and holding them until the Apple logo appears.
Let us know how you get on.