Try force closing the calendar app by double-tapping the home button, locating Calendar, swiping up on the image above the app icon, then tapping the home button again. Then test your calendar again. If you still can't add events, go to Settings>iCloud, turn Calendars off, choose Delete from My iPad (they will still be in iCloud), then after a few seconds, turn Calendars back on and your calendars will reappar on your iPad. Try adding events again.