In the past week, one two different occasions, I've had folders go missing from my Lightroom folders.
Each time, it has wiped everything back to a folder from May 17. The first time it was 2 folders, and the second time, those two folders plus two more.
The folders are still in there showing they imported to the computer, but the newer ones do not show that they have been imported to Lightroom or ever been edited.
Up front, I have moved files myself or changed there location. In the latest instance, I worked the files this morning around 7 a.m. and just sat back down to do some more work and they were gone.
Typo, I have NOT moved the files. I didn't do anything to the basic files. Sorry about that.
A possibility is that you have accidentally switched from one catalog to another.
Go to File->Open Recent and open those catalogs shown there and see if you can find one with the photos in question.