Sometimes applications in OS X stop responding, and you either have to wait for them to start working again or you have to force quit them. I recently noticed another application status called "Paused," when I brought up the process list in the Activity Monitor. OS X will pause applications when your system runs out of resources to keep them going. Ideally, they'll resume on their own, but they don't always do so. Fortunately, there's a way to manually resume paused processes. First, open the Activity Monitor and find the process ID (PID) of any paused processes. Then open the Terminal app and type the following, replacing 250 with whatever PID you need to resume:
kill -cont 250
Your application should be back to normal within a few seconds.