What kind of machine is this on?
Do you get the alarm at any feedrate or can't you move Z at all?
The Z axis has a motor with a built-in brake, possibly the brake has failed and won't release. If this is a solid-way machine, you may have a stuck gib. There could be a problem with the wiring for the brake or the motor. There could be a Acroloop card problem.
Many things need to be checked to determine the cause, send SportyBob a pm, he may be able to help you out.