What do I do if I get this when I try to open my Photos Photos cannot find the System Photo Library named “Photos Library.photoslibrary”

Moneymed wrote: Have I lost all of my photo's.

Probably not. Are you using macOS 10.11 El Capitan like in your signature?

You haven't told us much about what you're doing. Are you using iCloud Photos to synchronize Photos Libraries on your devices? If so, have you looked at iCloud.com to see if your pictures are there?

Have you used any "cleanup" apps on your Mac? They sometimes move things to the trash unexpectedly, or they can corrupt the Library. In any case, don't do that. But try looking in the Trash for the Library-- several people have found the Library there.

Or you might have accidentally moved the Library, and Photos lost track. Try doing a Finder search like this:

Search for "photoslibrary" and, if it's found, look at the bottom to find the path to the Library.

If you look around and can't find the Library, try restarting the computer.

Let us know what you find-- and if you need help, please tell us more about your system and what's going on...

Have you recently moved things around or deleted them?

Quit Photos, hold down the option key and start up Photos.

Does your library show up there?

Create additional photo libraries in Photos on Mac - Apple Support

You can use a finder search to see if it can find photoslibrary.

Where are you keeping your Photos Library?

By default the Photos.app will create the Photos Library in your Pictures folder.

To search for your Photos Library open a Finder window and enter the keyboard shortcut ⌘F to open a search field. Then type .photoslibrary into the search field and select the search option "Kind: Photos Library" from the list of search results.