How much free storage do you have? Apple recommends 10% free, and less causes all sorts of problems. I saw really crazy things with 130 GB free on a 1 TB MacAir, and they cleared up when I transferred a bunch of stuff to an external drive. So, in my experience, even less than 15% free is problematic.
If your storage is OK, then try these things:
- Restart the computer (of course)
- Re-start in Safe Mode. This bypasses certain potentially disruptive processes, and it often helps. Safe Mode is different for different computers, so see this:
Start up your Mac in safe mode - Apple Support
- Make a new user and see if the same problem recurs with it. This is a bit harder, because a new user can't access a different user's stuff. You would need to move your Library out of your own user's folder up to the general "Users" folder.
Rebuild your Photos Library--close Photos and re-launch by option-command-clicking the app icon. If you're using iCloud Photos, this could cause Photos to start an hours-long re-sync with iCloud.
The idea is to find out if the problem is maybe with the Library, with your account, or with caches and login items, and stuff like that. One of these steps may fix the problem, but their failure to work also helps figure out what's happening. But, frankly, if the free storage is low, then nothing much will help.
Let us know what happens…