Restart the iPad: Hold down the power button until the power off slider appears. Slide it off. When the iPad finishes shutting down, hold down the power button again until the Apple logo appears.
If that does not work try a Reset: Hold down the power and home buttons until the Apple logo appears. The iPad will reboot. This usually takes a bit longer than a restart.
If neither works, make sure you know what your account settings are, then delete and re-add the account on the iPad. All information is stored on the servers, so don't worry about losing anything on the iPad.