The zero proximity detector on the Z axis probably has a chip of metal stuck to it or it is in some other way malfunctioning.
When the axis homes it moves negative until the proximity detector is activated then it moves positive until the proximity detector goes in-active and finds its zero.
If there is a chip or something on the detector it never goes inactive so the axis just keeps moving positive.
You probably can move it without zero. Go into Settings and turn ON Setting 53 JOG W/O ZERO RETURN. Be careful doing this now you can run the axis into the hard stops if you get carried away.
An open mind is a virtue...so long as all the common sense has not leaked out.