Photo Library

It can be distressing to find whole batches of photos have vanished from your Photos folder.

See if this helps: https://support.apple.com/en-gb/HT205966

Have you tried typing Photos Library in Spotlight Search, or perhaps just Photos?

They may have been moved to the Cloud. If you have an iCloud account, if you have an iCloud account, You don't actually need to be using it, but if you have, they will be marked as yellow in the bar at the bottom of the iCloud window, like this:

Click on the Manage button and you might see something like this:

The above screenshots are from my personal iCloud account.

Where is your library located? If on an external HD how is that drive formatted and connected to your Mac?

Have you tried launching the library with the Option+Command keys held down and repair the library?