Is there a fuse anywhere in the system? Often when a machine stops working suddenly, it can be traced to a blown fuse. The next most probable thing is a wire that's come loose. If that's not the problem, then there are lots of other things that need checking, like the computer you're using (have you tried this with another one?) corrupt software (have you tried re-downloading and reinstalling Mach3?) If you've only run short G-code programs, it might be that you never had a valid license for Mach3, and it cut out at the 500 command limit. (Have you tried restarting everything and commanding a move?) If you're using a cheap crappy breakout board, it could have suddenly died - you'll see shortly if the new one works any better. It could also be a failure of the power supply.