what to do when your photos suddenly disappeared on your phone?

Please check the following

1. If you have restored your iPhone with a backup that does not contain the Photos that you are looking for then please look for correct backup on your PC/Mac --> Locate backups of your iPhone, iPad, and iPod touch - Apple ...
2. If you have restored from iCloud backup, then note the following
   1. iCloud backup doesn’t include information that is synced and stored in iCloud, like Contacts, Calendars, Bookmarks, Mail, Notes, Voice Memos, shared photos, iCloud Photos*, Health data, call history, and files you store in iCloud Drive.
   2. *-If you had turned on iCloud Photos, your content is automatically stored in iCloud. 
3. Probably you may have un-synced photos with Mac / PC, sync them again
   1. For PC: --> Sync photos in iTunes on PC with devices - Apple Support
   2. For Mac --> Sync photos between your Mac and iPhone or iPad - Apple ...
4. Have you ever imported your photos to Mac/PC and used Delete after Import --? Transfer photos and videos from your iPhone, iPad, or iPod ...
5. Do you have a backup (iTunes) --> Restore your iPhone, iPad, or iPod touch from a backup ...
6. Have you ever uploaded them to any Cloud Drives (iCloud Drive, Google Drive, One Drive, Dropbox, Flickr, box, Amazon Drive, Adobe Creative Cloud etc.?)
7. At last but not least, check the Apple ID that you have signed in to your device.

when photos suddenly appear, it might mean because you forgot to log out of your apple id and someone else has been taking pictures with your account. just delete it, and if you remember what device you used, ask for it back to log out again.