The apple photos app has been deleted by accident. How can I restore the app?

According to the following, the Photos app isn't one of the apps that can be deleted. Have you tried using Search (swipe down from the middle of the home screen) to see if that will locate it (i.e., is it possibly accidentally moved to a folder or other home screen)?

See: Delete built-in Apple apps on your iOS 12 or later device or Apple Watch - Apple Support