Photos app not visible
That’s not an app that’s designed to be removed, so it’s either hidden and you can use spotlight to search for which folder it moved to or the database that records where apps are visible is corrupted. 


https://support.apple.com/en-us/HT204221


For the former you want to be sure you have a good backup and then restart the device to see if Spotlight can rebuild. If there is no free space, remove some content and restart again. 

For the latter, get a good backup and restore the device.