Hi,
You didn't mention what version of iMovie you are using. Assuming that you are using iMovie 10, try opening iMovie while holding down the Option and Command keys and then selecting to delete preferences in the box that appears. Sometimes that cures glitches.
You also can try looking in the iMovie backups folder. To get there follow this file path from the Finder menu: Go/Home/Library/Containers/com.apple.iMovieApp/Data/Library/Caches/iMovie Backups. When you get there you will see a list of backed up iMovie libraries. Click on one that has a date just prior to when you lost your project. iMovie will open in the backed-up library, and you can then check in the Projects browser screen to see if your project is there.
-- Rich