This error message means that Mach3 is receiving a signal that indicates there is an active emergency stop in place. 

If you have a physical E-Stop button and it is NOT pressed, you can change the ActiveLow setting by going to Config>Ports and Pins>Input Signals and change the ActiveLow column for that input signal. Restart Mach3 and manually press the E-Stop button to ensure it functions properly. 

If you do not have a physical E-Stop button, make sure the E-Stop function in Config>Ports and Pins>Input Signals is not filled out. If it is, disable it and change both the Port and Pin assignment to zero. If the E-Stop is still active after restarting, change ActiveLow setting for E-Stop. 

If this error message appears intermittently, you may have noise issues. Electric noise can cause the signal on the E-Stop pin to briefly go active. Noise can be filtered to some extent with Debounce, but it is usually preferable to address it in the hardware with proper shielding and grounding. Refer to this forum article for more information on excess noise:

http://www.machsupport.com/forum/index.php/topic,25616.msg180583.html#msg180583