Two ways forward. (From what you've said above I'm assuming you are using iCloud photos.)
1 - Hold the option key down while starting photos. It should list the libraries on your system. If so, try opening them one at a time until you find the one with your photos (Let us know what happens)
2 - If that doesn't show any libraries (or you just want an easy life)
Create a new library (Same option key trick when starting, but just select "create new"
Put it in your pictures folder and call it photos library.photoslibrary
Open it, go to photos preferences (general tab) and click "use as system photos library"
Then go to the iCloud tab, and turn on iCloud photos.
Leave your mac on, with photos open and wait. It might take a while, but it should download all the images from iCloud into your new library.