Hi, andreanne128,
Try deleting preferences. It is a safe procedure. To delete preferences, open iMovie while holding down the Option and Command keys and select to delete preferences in the box that appears. iMovie will open in a new library. Reopen your old
library to get back to your projects. See if that cures it.
Another thing you can try is to copy your media into a newly created project and see if that cures the problem. To do that, do an Edit/Select All, Edit/Copy, of the media in your project and then Edit/Paste it into the newly created project.
-- Rich