I'm branching your post to the new thread. You problem is not the same as the original poster of this thread.
What version of LR where you using before you installed LR Classic CC 7.2?
I suggest you find your original catalog file (Hopefully you know where it is located on your hard drive) and double click on it to open it and LR.
If it is from an older version of LR, LR Classic CC will need to Upgrade it to work with LR C CC. There is nothing you can do about that. You will need to let it complete.
The catalog file end with .lrcat so if you don't know exactly where it is on your drive you can use OS X Spotlight to search for all files ending with .lrcat.
I used version 7.1. of Lightroom. I have opened the catalog from the hard drive and it still does not open. When he asked to update the catalog I left the computer doing that operation for 48 hours and it could not be updated. If I lost my catalog for an Adobe update I want to claim. I can not waste my time and my money this way.