Your Photos Library should be in the Pictures folder where Photos can find it. Sometimes, with an accidental flick, a Library can get moved somewhere else, maybe even the trash, so you may need to look around. You can use Finder's search field to hunt for a Library's exension, ".photoslibrary," (include the leading . in the search.)

The path to the selected find is shown at the bottom.
A Photos Library can't work in the trash or in the iCloud Drive folder or in some special use folders, so if you found it in a crazy place, you should move it back to the Pictures folder. Once it's settled, you can double click on it to open it in the Photos app.
If you are using iCloud Photos to synchronize your Photos Library, then a safe option would be to create a new empty Photos Library, designate it as the System Library, turn on iCloud, and wait for it to fill. You can see this:
Make a New System Library connected to iC… - Apple Community