Open the Photos app
Tap Albums
Scroll down to Utilities
Tap Hidden
Authenticate with Face ID, Touch ID, or passcode if prompted
View the hidden photos and videos
To unhide a photo, open it, tap the Share button, then tap Unhide
To show the Hidden album in Albums, go to Settings
Tap Photos
Turn on Show Hidden Album
If you still cannot find a photo, check Recently Deleted
Open Photos
Tap Albums
Scroll to Utilities
Tap Recently Deleted
Authenticate if prompted
Restore any deleted photos if needed