When you turn on the G540 does the X axis motor 'thump' and lock up? If no, then it is not getting DC power from the G540. If it is locked you won't be able to turn the shaft coupler by hand.
If it is locked but just doesn't respond to jog command, it is not getting step signals from Mach3 due to a broken wire or bad connector pin in the parallel port cable. Mach3 configuration could have changed to disable the X axis. Check the related configuration settings. The G540 .xml file could be corrupted.
CarveOne
http://www.carveonecncwoodcraft.com