Disconnecting the computer will give you the problem as your limits are most likely normally closed switches, so when you disconnect the machine from the computer you are breaking the circuit and Mach sees it a limit being triggered.
To check that the port is still being seen correctly you can go to Config menu then Ports and pins the Inputs and change the active state of your limit switches, this should now allow you to reset Mach. If it does then either your active state got changed some how or there is a problem in the wiring or port, the only real way to test would be with an ohm meter if visual inspection is not finding anything.
Hood