Perform a "reset" on the device by pressing down and holding the home button and the sleep/wake button for up to 30secs. until you see a black screen with the Apple logo, then release.
Then test to see if it solved the problem.
Close the App Store app completely and then reboot your iPad. Try again after that.
Double tap the home button and you will see apps lined up going left to right across the screen. Swipe to get to the App Store app and then swipe "up" on the app preview thumbnail to close it.
Reboot the iPad by holding down on the sleep and home buttons at the same time for about 10-15 seconds until the Apple Logo appears - ignore the red slider if it appears on the screen - let go of the buttons. Let the iPad start up.