The system should be wired so, if there is an axis drive error, Mach3 should go into EMG stop and the drives should get de-energized. It all depends on your EMG stop circuit wiring and how you want drive errors to be treated in your system. Sometimes you should start the program from the beginning after a stop, it depends on which part of the program is was running at the time, most of the time the part being machined gets ruined.
Regards,
kreutz