It could be any number of things: blown servo drive; damaged servo motor; failure to release holding brake in servo motor; physically jammed head or screw; etc..
Power up, and without bothering to try to home, switch to Slow Continuous jog mode.
Can you jog X and Y in both directions?
Can you jog Z down?
If you are sure Z is clear of the switch and the end of travel, can you jog Z up?
Are all of the Green LEDs on your servo drive unit lit?
Do you know whether your Z servo motor has an integral holding brake? If you are not sure, see if there are any wires connected to the two brake screw terminals low on the left edge of the drive.
If you can turn the Z motor or screw with the power off (e.g. with E-stop pressed in) see if you can jog Z successfully after moving it a little by hand. If you can, but it stalls again in the same spot, then you probably have a burnt spot on the commutator of your servo motor.
Let us know what you find out.