Can you help with more detail? For instance, what happens when you try to play an old video? Do you see it in the Thumbnail view? Will it expand when you double click on on it? Will it open in Edit? Do you get an error message? What is the format of the video file-- mov, m4v, etc? Does this happen with all videos, or just old ones, or just some of them? Can you export a video file, and will the export play?
Where is your Photos Library stored? is it in your Pictures folder or on an external hard drive? If on an external drive, how is that drive formatted? Do you use iCloud to sync your devices? If so, do you have "Optimize Mac Storage" checked in Photos preferences? Is this your system library? Do you have "Copy Items" checked in Photos preferences?
Have you tried these things?
- Restart the computer (of course)
- Rebuild your Photos Library--close Photos and re-launch by option-command-clicking the app icon.
- Make a new user and see if the same problem recurs with it
- Use another library if you have one, or make a small test Library with a few pictures in it and see if the same thing happens.
- Start in Safe Mode (different for different computers) This bypasses certain potentially disruptive processes.
Start up your Mac in safe mode - Apple Support
The idea is to find out if the problem is with the Library, with your account, with particular files, or with caches and login items, and stuff like that. One of these steps may fix the problem, but their failure to work is also diagnostic.