Check the z axis limit switch. If you go to (Display) | (Diag) you can look at the different inputs the machine sees. Check and see if there is a 1 on the Z axis limit switch. If that switch is high it will estop the machine, and if the machine is e-stopped I think it will show external fault on the VFD. If it's a stuck limit switch, I think you can press and hold [RESET] and temporarily engage the drive so you can move them around.