The images are never "in" Lightroom - they are still there on your hard drive. Lightroom just keeps pointers to them in the catalog, which is what you "import". Plus of course any adjustments you have made.
So don't worry. They're still there. Even after reinstalling the application, your old catalog is there, just go look for it and double-click to open it. The default location is in "My Pictures" (but you can put it wherever you want).
It's confusing as there seem to be 4 catalogues. I'm not sure why the system was using the wrong one.
First the application doesn't contain photos. The catalogs have links to files on your HD so the removal of Lightroom will not have affected the files.
Now they may have removed the Preference file which will cause the loss of which is the latest catalog.
Go to Finder and search for files ending in .lrcat, they are the catalogs of your work. Double click each one in turn by double clicking (start from last modified date although that will most likely be the one with the "missing files") and work down the list until you find the one with what you are missing. It may well be the second or third...
If you back up the catalog frequently be aware of the back up copies that will be there