Bandit, do you have the wiring diagram? If so, look for the E-stop "string". It's called that because the components in the string are all in series, so the power has to pass through them in order. If any of the components fails, the string is broken and the machine goes into E-stop.
Typically, the components might be:
• the E-stop pushbutton switch or switches
• the axis limit switches; often only one per axis, with a dog to trigger it from each direction
• thermal overload relays for the motors, usually mounted under the contactor
• certain circuit breakers for important circuits (not for a worklight, for instance)
• a pressure switch, for low hydraulic or air pressure
It could even be a bad power supply that powers the E-stop string. The voltage coming out at the end of the string should be the same as whatever goes in at the beginning. So, if the voltage is low to begin with, it's treated the same as an interruption by one of the components.
The string could be something like 120v AC, or 24v DC, which is why the electrical diagram is so important here. Also, the the diagram should show the wire numbers that connect each of the components, so you can find that wire number in the back of the machine and measure the voltage at each point where the power goes through a component, then the wire number is changed as the power then goes to a new component. If you don't have the diagram, I think you're gonna need help from Hardinge.
The spindle drive or servo drives could also cause an E-stop, but I'm going to guess that neither is the problem, because you would likely also have an alarm on the display, or an alarm indicator on the drive.
Good luck.
mt