Lightroom all of a sudden says "This file could not be found"?

Photos aren't contained in the catalog, their locations and adjustment settings are, but the photos themselves are just on the hard-drive, somewhere.  If something else, outside of LR deleted those photos or moved them, then LR wouldn't know that, and indicate they are missing.

Are the photos still on your hard-drive or not?  And what is an example path where LR thinks they should be?

Are they in a different location on the hard-drive than they used to be?

When you Import photos into LR it remembers where they are, but if you do something to them outside of LR, not using LR's Folder Panel, then it will still expect them to be at the Import location, and if they are not, then you'll see question marks or exclamation points with a message about them not being where they originally were.

One mistake that could have been made, is that you imported photos into a LR catalog and that catalog remembers them one place, then you opened a new LR catalog and did a Move import, which would move them some other location and they'd be remembered in that second catalog correctly, but not in the first catalog, anymore.  If you only have one LR catalog then this is probably not what happened.