It looks like your best choice is to do an OS restore through iTunes if you can. If that does not work, I'd contact Apple Support.
Also, for the record, videos that you take using the Camera app do not appear in the Videos app, they appear in the Photos app as well (strange, I know).
To Rebuild the Photos.app Thumbnails:
Backup your library before following these instructions.
Delete the old thumbnails and various parts from the library:
Quit the Photos app.
Open Finder, and go to the Photos.app Library, on my system it is called Photos Library and is found in the Pictures folder.
Now right click the library and click Show Package Contents.
Next delete the folders called Previews and Thumbnails.
Then delete the files called ImageProxies.apdb, ImageProxies.apdb-wal, Library.apdb and Library.apdb-wal from the database folder.
Lastly delete the folders RKCloudResource, RKImageProxyState, RKMaster and RKVersion from the recovery folder.
Now rebuild the library
Open the Photos app while pressing ⌥+⌘. It will ask you to if you want to repair the library, click Repair. It will do a repair, then your photos will go through a reprocess/reimport process. Once finished the thumbnails will start rebuilding.
P.S. I tried this on my own Photos library, and it worked.
Best Answer
Looks like it's not possible according to the Apple pagE: https://support.apple.com/en-us/HT207023