I'm having the same problem. Yesterday I was importing photos to my macbook pro (mid 2014 retina, OS X 10.10.5) in halftime brake of a basketball game. I was on an assignment and usually I send some pics during the brake. Suddenly after importing the photos Lightroom stops responding. After that it has stopped responding or has worked very very slowly every time I open the app. I was so freaking frustrated! I'm happy that the match wasn't very important so I could send the photos from my old computer when I got home.
But this problem has to be taken care of!