There are several reasons for spindle not turn. Possible causes for spindle failure:

Inverter is off or has an error .
· Locate the inverter then look for the display on the inverter.
· If the display shows nothing, ensure there is power to the inverter. Commonly, inverters are shut off via a large knife switch and sometimes they are left off by accident.
· If there is power to the inverter, it is possible the power is insufficient or too high, or that the inverter is no longer functioning.
· If the display is active, look for an error message.
· On an Altivar brand inverter, if the display reads anything other than "RDY", there is likely an error.
· Likely Altivar errors include "OCF" "OSF"
· On a inverter, if the display reads anything other than solid or blinking numbers, there is an error. Likely errors are "OCC" "OCD" "OCA".
With either brand of inverter, if the error starts with the letter "O", that means there was an overcurrent failure. If this is the first occurrence, reset the inverter by powering it down, waiting for a few minutes for power to completely dissipate, and then powering back up.
If the error persists, contact tech support.

Toolchanger sensors are misaligned
If the machine employs an automatic toolchanger, it is possible the spindle cannot sense the tool, which causes the spindle to refuse to spin.