Close the calendar app and reboot the iPad and see if that fixes it.
Double tap the home button and you will see apps lined up going left to right across the screen. Swipe to get to the Calendar app, then swipe "up" on the calendar app 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.