cannot open photo library

Perhaps some more information would help:

• Are you using iCloud Photos to synchronize your devices?
• Do you have “Optimize Storage” checked in Photos’ Settings
• Where does your Photos Library reside-- in the Pictures folder of your internal drive, or on an external drive? or on a network? on iCloud drive?
• Has this problem always existed, or did it just start? Can you think of anything that changed that might affect this?

What have you tired? Have you tried these things?

• Restart the computer (of course)
• Re-start 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 

Let us know so exactly what you're doing so we can help…

And what were those changes?