Restoring Catalog after a reinstall

Did you keep at least one backup of your catalog on a hard drive that did not get clobbered by the computer repair person?

Did you also keep at least one backup of your photos on a hard drive or cloud (not clobbered by computer repair person)

/Followup/ Immediately I just made an assumption, that your catalog was on your root drive, your C drive. Should have asked, was it?

With a fresh install, Lightroom might default to creating a new (empty) catalog, typically in your Pictures folder. You have two options:

1. Within Lightroom Classic while it's running, choose File > Open Catalog, and browse to the location of your real catalog.
2. Outside of LrC, without it running, browse to the location of your real catalog file and double click it. This will launch LrC using that catalog.

You say "new catalog".  You need to open the catalog that was in use before you did the system repair. If that file has been lost, you need to use the last backup of the catalog that you have.  The catalog is where LR stores all the information about your files, including where they are stored, and the edits that have been done.