It sounds like you're using iCloud Photos to synchronize your devices-- is that correct? Have you checked to see that all your pictures are at iCloud.com? If all your pictures are there, then you're safe! You just have to find them on the Mac. Have you looked to see if the Library is in your iCloud Drive folder? A Library can't work from there, is if you see it, drag it back to the Pictures folder.
You can check to see what Libraries are available and where they are by closing Photos and then option-clicking on the Photos icon. You get the Library Chooser that looks like this:

Perhaps you can find the Library that way. The normal place is the Pictures folder, but sometimes things get dragged around while we're not looking.
If you don't find it, then try these things, and search again:
- 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
If the Photos Library is gone, then the next step is to create a new Library, designate it as the System Library, connect it to iCloud, and then let it fill. See this:
Make a New System Library connected to iC… - Apple Community
Let us know how it goes and what you find…