Z Axis Stepper Motor Will No Move the Axis

Create a Parameter list ($$ command, copy to a text file) which will answer a few questions. Add that to your dropbox.
When you say Z axis mechanically moves smoothly, do you have a way of manually rotating the Z Stepper ?

One possibility is obviously a bad stepper.
Did you check A and B winding continuity with voltmeter?
So lets say the X stepper is working properly.
You could swap wires from the Z motor and the X motor at tinyG end, just to see if perhaps a driver is not functioning correctly on tinyG.

How many steppers are there, 3 or 4?
I can only see what looks like a stepper at end of gantry.

Firstly, good job at placing your expensive electronic device on an earthed metal conductor. Bonus points for magic smoke!

Second, have you adjusted the drivers power? Have you connected that stepper to a different axis? What happened when you did that?

Can you disconnect the Z motor from the load and check that you get same result unloaded?

$$ listing would help maybe

I figured out my issue. It was the Travel per Revolution ($4tr in my case). It was set to 360000. The others were all 1.2500. Once I changed that, it worked as it should. This value either persisted after I ran the factory default command ($defa=1), meaning the factory default doesn’t change that setting or more worrisome, that that is the default value.

Either way, I’m back in business.