Recover deleted photos on iMac?

[dragonfly]

Please help ....

A couple months ago I was clearing out old photos and I did something wrong. My Photos app gives me this message: 

Photos cannot find the System Photo Library named “Photos Library.photoslibrary”.
You can open another library and set it as the System Photo Library in Preferences.

I can't find anyyyy of my old photos, especially photos of my pets, including my beloved little kitty who I lost in my divorce, dating back to 2010 and some from even before that. I can't for the life of me imagine that I would have deleted the entire folder ... is there a way to re-download / restore the library.photoslibrary file?

Any file recovery software that could find and restore them?

I'm beside myself.

The files are not in my trash.  thanks in advance ...

[BlackCurtains]

Do you run Time Machine?

[dragonfly]

Nope