Welcome to iMore! Before restoring your device, try this:
1. Assuming your using an iOS device, open the Calendar app
2. Tap on Calendars at the bottom of the screen
3. Tap Hide All Calendars
4. Hard reset your device by holding the power button and home button down simultaneously until you see the Apple boot logo
5. Open the Calendar app again
6. Tap on Calendars at the bottom of the screen again
7. Tap Show All Calendars
This usually forces a refresh that should kick things into gear. If you're still having issues let us know!