Check your Trash and see if you see the iMovie app. If so, right-click it and select Put Back. Then restart your Mac. May not work, but it is worth a try.
If that does not work, you can purchase a new copy from the Mac App Store for about $15.
If you have an older version of Mac OSX that does not include the Mac App Store, you may be able to find copies if the iLife 11 Retail Package on Amazon or eBay.