It's possible you restarted the simulator too quickly, without allowing enough time to close all the opened files and programs, and this will make GSX not starting the 2nd time. Restarting Windows if this happens will surely fix this and, in general, you must allow some time before starting the sim again.
Try the GSX Control Panel with the simulator is open. You'll see the Compile button saying the sim is still running. Then exit the sim, and take notice how much time it takes for the button to come active. That's the sure way to know it's safe to restart a new session.
