If the Z-axis is only going up and not down, it is likely that your Z-limit sensor has failed or is malfunctioning.
Check that the little red button on top of it presses and clicks properly and is in good condition.
Did you operate the adjustment wheel with the machine at zero point? Because if the machine is at the zero point and you manipulate the wheel more than a quarter of a turn, it will sink into the sensor.
For each “big” manipulation of the wheel, it is imperative to move the head up at least a few millimeters.
If it fails and/or no longer clicks, it must be changed. You can find the part on theonline store. However, it will be necessary to re-solder the threads.