Where were those photos stored? On the local hard drive (Mac drive)? On an external hard drive? In the first case there's no way to restore them unless you still have that drive and you connect it externaly to your computer. But if you did not backed up your catalogs, even if you recover the photos you will have to procees them once more. It's always wise to store your photos on external drives and even make duplicates of those drives. There are inexpensive external drives rack that take two drives and duplicates on the second drive everything you copy on the other drive. Besides, storage has become dirt cheap these days.
PS. I even store my photos on DVDs, making a backup copy for every DVD with photos. You never know when the big one will come (solar storm, that is) and fry all the drives. Just kidding in the last phrase.
All of the files that you were working on and managing in Lightroom were not actually "in" Lightroom. Your files are stored in folders on your computer. Lightroom simply points to those files and displays them. Reinstalling Lightroom would not have any impact on your image files unless the hard drive(s) were reformatted in the process. Lightroom uses a catalog which is a database to keep track of where your images are located. It also stores all of the adjustments made to your images. If you backed up that catalog before reinstalling your software, and if you still have all of your images, then you should be able to open that catalog that you used previously and have all of your work still in place. If you don't have the catalog then I'm afraid you will just have to start over. Whether or not you still have your image files isn't clear.