can't see old photos library on mac

Anusca wrote: I've opened a new library and I can't find the the old one on Mac. thanks

Photos Libraries are usually found in the Pictures folder. If you don't find the missing one there, close Photos and option-click on the Photos icon to open a dialog that lists all the Photos Libraries that the app can find. If that doesn't work, use the find function in a Finder window to search for "photoslibrary" :

If it finds it, the path to its location will be shown at the bottom. You can open it by double clicking.

Let us know what happens...

Launch the Photos app with the Option key held down and you'll see every library on your Mac in the next window. Just select the one you want: