Welcome to the Lightroom Classic community. I'll need more information to help you with this.
What version of Lightroom Classic & the operating system are you working on? Where is the catalog stored? Is it on the internal drive or an external drive? Do you have Onedrive backup enabled on your computer?
What troubleshooting steps have you tried?
If your catalog (.lrcat) is stored on a external drive, please copy/move the catalog to the internal drive/desktop & open the catalog (.lrcat) directly.