A number of people have reported problems with Photo > Photo Merge in CC 2015.2, with no workarounds reported:
Panorama and HDR preview blacked out?
Do Help > System Info to find out exactly which version you are on (don't rely on your memory). If you are on 2015.2 or 2015.2.1, your best bet is to do what many, many others have done and revert back to the last stable release, 2015.1.1: http://www.lightroomqueen.com/how-do-i-roll-back-to-lightroom-2015-1-1-or-lightroom-6-1-1/