Have you taken any backup before reinstalling?
By default, Lightroom stores the catalog in your Pictures>Lightroom folder.
You can also perform a search for the catalog using .lrcat (extension of Lightroom Catalog file).
If you reinstalled OS X then it wiped the drive of all files which removed the LR catalog file along with any and all images you had on the internal hard drive.
So if you didn't make a backup of your files, including the LR catalog file, they are gone.