I have to say that my first course of action would be to go back to the old computer and Mach3 if its still available.If not,I hope you kept a copy of all the parameters that related to the machine and can find a way to make them work in Arduino land.Have you tried rotating the steppers by hand-with the power off?If it isn't a mechanical issue from years of inaction and the pinouts are correct then I would move on to the wiring in case mice have been at it and if not then I would suspect the wrong pairs of wires on the steppers being connected to the right outputs of the drivers.
For help with GRBL there is a lengthy thread about it's latest developments and thats where to look for an expert.