Serious but mysterious z axis issue with my Wanhao Duplicator plus MK2.

[Heera]

I've got a serious issue with my printer that I can't seem to solve. I'm a pretty long time lurker in the 3d printing space, I understand the fundamentals of 3d printer design to a pretty high level. But this one has me stumped.

I gifted a new Wanhao duplicator i3 plus MK2 Speed Test Scrabble Word Finder Solitaire to a friend, the one with a proximity sensor for Z height stop as well as ABL.

It's been working well for a month since I got it, kids printed a small toy, then sent a command for another, but suddenly for no apparent reason a problem emerged, The kids next toy was simply extruding spaghetti in mid-air.

Z height refuses to zero. When I home or use menus to lower the extruder, it starts to rise up. And It keeps going up till some Arbitrary height above the bed.