Are you using iCloud to sync your calendars, or a different service (such as Google or Yahoo!)?
Either way, go to the online site on a computer and check to make sure your events are correct there. If they are you can:
Go to Settings > Mail, Contacts, Calendars, open the correct account, and turn off the Calendars. Tell it to delete local events when prompted.
Go ahead and open the Calendar app to be sure it is now empty.
Go back to the account settings and turn the Calendar syncing back on.
Open Calenars (after waiting a bit) and check to make sure everything synced back correctly.
Don't forget to tap on the Calendar button at the bottom and make sure all your calendars are checked, so that they show up.
Note: you can check iCloud syncing by going to icloud.com in a computer browser and signing in with your iCloud ID.
WARNING: If your website (online) account shown on the computer is not up to date, do not do this. You will lose anything that is not online.