Photo app will not display photos

Wow-- that's kind of the primary purpose for the Photos app!

First, some questions:

• Is your Photos Library located in your Pictures folder? or do you use an external hard drive?
• If you see the Photos Library in Pictures, what happens if you double click on it?
• If you close Photos and option-click the Photos icon, do you see more than one Library listed? I have several:

 The first things we usually try first are these:

• 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 

• 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. You can create a new Library by closing Photos and then option-clicking on the Photos icon.
• Rebuild 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.