disappeared pictures fro photos app. in imac

If you have your account name showing in the Finder Sidebar, select it, then the Pictures folder, and then double click on Photos Library to see if it will load.

If that doesn't work, with the Photos Library selected, enter Time Machine, go to a date where the photos showed, and restore the Photos Library.

Have you checked the Recently Deleted album in the Photos application? If you were syncing photos with iCloud, have you checked iCloud.com/Photos using a computer browser to see if the photos are there or in the Recently Deleted album there?

Do you have a backup you can restore from?