Hi Moritzzz,
Were the video clips in an iMovie project? How did they get lost?
iMovie keeps a backups folder on your Mac. To get to the iMovie Backups folder, follow this file path from the Finder menu:
Go/Home/Library/Containers/com.apple.iMovieApp/Data/Library/Caches/iMovieBackups
When you get there you will see a list of previous backups of your iMovie library. Click on one dated just before you lost your videos. iMovie will open in that library. Navigate to your projects browser (where your projects are displayed as icons) and see if your projects are intact. If they are, your videos should be in there.
Your other option would be to use your TimeMachine backup, if you keep one, to restore your iMovie library to a point in time preceding the loss of your videos. Restore it to a different location than the present library to avoid overwriting it. You would lose any work done from the restoration date to the present.
Absent a Time Machine backup, if you have backups of your iMovie Library anywhere, you might check those.
-- Rich