That is strange. The Photos.app is a part of the system and protected by the System Integrity Protection, and it is nearly impossible to delete the Photos.app.
Could it be that nit the Photos.app has been deleted, but the icon has been removed from the Dock or an alias on the Desktop?
You can always find the Photos.app in the Applications folder and just drag it from there back to the Dock.
Your photos are not stained in the Photos.app -they are stored by the Photos.app in a database, called the PhotosLibrary. By default, you will find the Photos Library in the folder Pictures.
You can see all Photos Libraries on your Mac, if you launch Photos while holding down the options key ⌥. Release the key ⌥ , when the dialog to select a library appears. Then select the largest library to find your photos. Try the libraries shown all in turn.