Photos reappeared on mac

What has changed, directly before the photos reappeared?

• Did you install a system update on your Mac? which system version is running now?
• Did photos crash or show a message, that the library needs to be repaired?
• Did you turn on a mobile device, that you have not used in. a long time?

I am asking about a system update, because Photos may switch to a different, older Photos Library after a system upgrade, if your main Photos library is in an unsupported location, for example on a NAS, or on a folder syncing with iCloud Drive. In that case you have to move your Photos Library to a compatible location, as described here: Move your Photos library to save space on your Mac - Apple Support

You can check, which Photos Library is currently used by Photos, by opening the Photos > Preferences (or Settings) and click the "General" tab. AT the top of the Preferences panel is a button "Show in Finder". Is Photos using the library that it is supposed to use? Where is your Photos Library stored?

I am asking about crashes or library repairs, because Photos may recover lost photos, when it is repairing a library. If Photos could not be deleted properly and are no longer linked to the thumbnails, they may be lying hidden in the library and reappear again, when Photos is repairing the library.

iCloud Photos may put photos back, when you turn on a device, that has not been used in a long time, where the photos have not been deleted.

Can you give us more details? Is this Photos? Did you have an old library lying around? Do you use iCloud Photos?

Did you drag a library onto the Photos icon in the Dock?