I'm a Mac, and I've never had this problem...
Crapintoshes. inb4Macusers :3
It will act as if its frozen, but your Console will be saying stuff and whatnot, if its your computer that's happened, try
Ctrl Alt Delete, to get the Task Manager out, and see if that works, if not, then do a LAN or just a Multiplayer with a password, if those fail you can try a Dedicated LAN, but I have no idea what those have.