Where is your library located? If on an external HD how is that drive formatted and connected to your iMac?
Try this: as a test launch Photos with the Option key held down and create a new, test library. Import some photos and test to see if the same problem persists. Does it? (This tells us if the problem is limited to your current library or is more wide spread)
Then try opening your current library. If that fails boot into Safe Mode by booting with the Shift key held down and check there to see if the problem persists. Reboot normally and test again.
NOTE: Safe Mode boot can take up to 10 minutes as it's doing the following;
• Verifies your startup disk and attempts to repair directory issues, if needed
• Loads only required kernel extensions (prevents 3rd party kernel/extensions from loading)
• Prevents Startup Items and Login Items from opening automatically
• Disables user-installed fonts
• Deletes font caches, kernel cache, and other system cache files
