If you're running GNOME 3 Shell with the MGSE extensions enabled, you probably have the faulty MGSE media player control/indicator extension enabled as well, and it's been found to cause GNOME Shell to stop responding properly after an indeterminable/somewhat random amount of time. If you look at this bug report
I filed some time ago, you can find a couple of patches that, when applied to the extension using the instructions provided in the comments along with the patches, should fix the extension and prevent it from causing the Shell to stop responding.
If applying patches isn't really your thing, you could always launch Advanced Settings, navigate to the Shell Extensions section, and turn off the Media Player extension, then force the Shell to restart using the 'r' command from the run dialog (Alt+F2) to ensure the extension is completely unloaded.
Linux Mint user since 2010, fluffy dragon forever!
Registered Linux User #473695