Which system version do you have installed? The upgrade to macOS 10.15 Catalina deprecated older video formats that are no longer compatible after the upgrade to macOS 10.15 Catalina or macOS 11 Big Sur.
Select one of the videos in Photos and look at the Info with ⌘i . What does the info show as the format for the video?
What is the filename extension shown? The codec? The frame rate? Look ias these places in the Info for the video:

if you jus t upgraded to a newer system version, you may need to convert the original videos to different format, after you export them. (About incompatible media in iMovie for macOS - Apple Support)
If you did not upgrade the system and are still using macOS 14 or older and the videos stopped playing out of the blue, you may want to check the following items:
- Have you moved your Photos Library recently to a different location? if yes, where is the library now?
- Have you been running any cleaning applications to save storage? If yes, which application?
- Are you using iCloud Photos and "Optimise Storage"?
- Do still have plenty of free storage on your Mac, at least 10GB?