Almost all my photo's have disappeared.
The individual files which I have named and dated are all still there but the folders are empty
iMac 21.5″, macOS 15.6
Please read the following user tip and restate your question/problem in detail:
Writing an Effective Apple Support Communities Question - Apple Community
Do you have iCloud Library enabled? If so do you also have Optimize Mac Storage enable.
Have you tried restoring the library from your Time Machine backup from just before the photos went missing?