The Photos.app may have switched to a different Photos Library.
You may want to check, which Photos Library is currently used. You can see this in the Photos > Preferences > General. Look for the Library location at the top of the pane and click "Show in Finder". Check the size of the revealed library, if it is large enough to hold all your photos. Look at the creation date, if it is your old library or a new one.
If Photos has created a new library, try to find your previous library with all your photos. Do you know, where the library should be? Perhaps on an external volume? You can also let Photos search for a different library:
- Quit Photos, if it is running.
- Hold down the options key ⌥ while launching Photos again.
- Release ⌥ when the dialog to chose a library appears. Select all libraries in turn to check where your photos are.