The reasons that Ender 3 Z-Axis is not moving could be printing scale issues, motherboard failure, or eccentric nuts problem. To fix the eccentric nut issue, loosen them and do tightening them. For avoiding the printing scale issue, loosen the screws on the lead screw bracket. In the case of only the Z-axis of your Ender 3 failing to home, we can immediately tell that the Z-axis endstop is the culprit, meaning that either there are problems with its wiring (especially likely if you have been working on your Ender 3 lately) or the endstop switch is not functional anymore. 7i3k.

ender 3 z axis not homing