It might be that the videos are in an older, legacy, format that is incompatible with iMovie running on Catalina or later. Here's a link that shows the compatible and incompatible formats:
About incompatible media in iMovie for macOS – Apple Support (AU)
If the above is not the problem, try deleting/resetting preferences. To delete/reset preferences open iMovie while
holding down the Option and Command keys and select to delete preferences in the box that appears. iMovie usually will open in a new empty library. If so, reopen your old library to get back to your projects. Now see if your videos will display and play properly. Possibly you may need to reboot to set the changes. Deleting preferences is a safe procedure that will not cause data loss or disruption to your project.
Also, be sure that your software is up to date.
-- Rich