Sorry to hear about the data loss.
#1 if you have Time Machine up and running you can potentially restore your work. If you don't have a backup, consider it. Any computer user should have a backup if they care at all about their data (Mac, Windows, Unix, doesn't matter).
#2 Update all the software on your computer (Apple Menu > Software update). iMovie is up to version 8.0.6 (Go to iMovie > About iMovie to check your version).
#3 The permissions on your events / projects folder may be wrong / damaged so iMovie can't save the changes you are making.
-- Make a new folder on the desktop
-- Drag ~/Movies/iMovie Projects and iMovie Events into the folder on the destkop
-- Click on that folder once and go to File > Get Info
-- Go down to the bottom under "Sharing and Permissions"
-- Make sure your name is in the list with Read / Write
-- Unlock the padlock
-- Click on the wheel / cog at the bottom of the window and click on "Apply to enclosed"
-- Put the iMovie events and iMovie projects folders back where they came from
If the issue persists move the following to the trash and restart the computer:
~/Library/Caches
~/Library/Preferences/com.apple.imovie7.plist or just imovie.plist
Hope that helps.