Find your Electrical Schematic for the machine. It will be important to help Diagnose the issue.

Pictured is a page out a Selexx manual for example.

In the Box is what you should look for. This is the Z stopper sensor.
The green arrow is the X input to the control. This will help you diagnose the issue on the next step to figuring out if you have a sensor or wiring problem.
252 is the wire number from the sensor signal to the control.

If a 24v signal comes directly out of the sensor the sensor is good. If it does not the sensor is bad. If 24v comes out of the signal at sensor and doesn't make it to the control you have a wiring problem.

Z AXIS STOPPER SENSOR