In fact, I noticed: Photos Duplicate Cleaner Photos.
John, have you been running Photo Duplicate Cleaner? Aggressive Duplicate Cleaners may damage the photos library, because some of them are removing the previews that are needed to access your photos and videos from the Media Browser.
Apple is warning against using this kind of duplicate cleaners: Using third-party apps to remove duplicate photos might damage your Photos for macOS library - Apple Support
Frequently it does not even help to try to repair the library, if it has been damaged by a spring cleaning attempt.
You could test, if iMovie will see the videos, if you create a new Photos Library with a few videos, and make this new library your System Photos Library and restart the Mac. Can iMovie see this test library?
This help page explains how to create a new library:
Create additional libraries
And this, how to repair a library:
Repair the library
If you current library is an iCloud Photos Library, I would not make this test, however. Switching to another System Photo Library will disable iCloud Photos and you would first have to ensure, that all photos and videos have been downloaded from iCloud, or you might lose some items. If you are using an iCloud Photos Library - iare you using "Optimise Storage"? Then you have to ensure, that the videos are been downloaded from iCloud in the full resolution, before yiu can access then iMovie.