I havent used Lightroom 4 in a while and after I clicked on the icon a "?" appeared over the icon. I went to the Creative Cloud menu on the top menu bar and clicked on it, ther dropdown window was blank (the window where I could install or reinstall Lightroom and other apps). Not sure how to fix this problem.
BLANK Cloud Screen http://forums.adobe.com/message/5484303 may help
-and step by step http://forums.adobe.com/thread/1440508?tstart=0