I have had this problem on Lightroom, 3, 4, and 5 on this one computer . . .
When I try to open Lightroom, I get an error message "assertion failed!"and then "Lightoom - Opening Catalog: Judy_1 copy.jpg.lrcat"
THe program locks up at that point and I can't use it.
I tried holding Alt-key and clicking on the Lightroom icon. It still will not open the program and gives me the same error message. It seems to be hung up on the catalog . . . which I have not been able to establish yet becuase I can't open the program.
Try to clear preferences and launch Lightroom. Please find the mentioned kb showing location of Lightroom preferences: http://helpx.adobe.com/lightroom/kb/preference-file-locations-lightroo m-4.html