What have you tried so far? I'm guessing you've rebooted. Have you tried re-starting in Safe Mode? This bypasses certain potentially disruptive processes. Safe Mode is different for different computers, so see this:
Start up your Mac in safe mode - Apple Support
Have you tried using another library? if you don't have one, you can make a small test Library with a few pictures in it and see if the same thing happens. You can create a new Library by closing Photos and then option-clicking on the Photos icon.
It could be something connected with your account. Make a new user and see if the same problem recurs with it.
You should try rebuilding your Photos Library--close Photos and re-launch by option-command-clicking the app icon.
The idea is to find out if the problem is 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.
Let us know…