While in iCal: File>Restore iCal...
Select the most recent backup.
What? You didn't backup iCal? 😱
You can try this
From the iCal File menu, choose Backup Database.
Quit iCal.
Navigate to ~/Library/Application Support/iCal/Sources.
Open each folder in the Sources folder and remove the file named "Index."
Open iCal; it will re-index your events.
...but I didn't find the iCal folder after a very brief search.