The two most common causes are:
1) The motor cables must be connected before any programming of moves occurs as the motors provide the position feedback information to the controller. There will be a generic "Motor Unplugged" error message displayed in this case.
2) A low battery supply can cause either an unplugged cable or tracking error fault on any connected axis. Verification testing should be performed with either a known fully charged battery or with the supply AC wall power supply.
Obviously, physically unplugging the motor after a programmed move or manual move has been initiated will also set an axis specific unplugged motor error.
If the fault still occurs with the ac wall supply- try a different Cat5 cable, then try the suspect motor on a different axis of the controller. If it continues to set the error with a known good cable- there is a problem with the motor and it will need to be returned for repair. If no motor will work on one particular controller port- the controller will need to be returned for analysis and repair.