Problem with Z-axis

Hi everyone,
I have a problem with the Z-axis of my Vertex Nano.
It printed really well many times, but since two days the nozzle
print far above the board. (0.35 on the screen)
I have recalibrate the homeoffsets (without any result)
Whatever I can do the printer will start the print normaly (drive.google.com/open?id=0B7lrx … lFoN1dsZFU)
But when the temperature is ok, the nozzle doesn’t gert down enough (drive.google.com/open?id=0B7lrx … lFoN1dsZFU)
0.35 as yiu can see on the screen.
What can I do ? (is there a way to reset the printer ?)
Thanks !

It’s normal that it primes the nozzle some distance from the bed.
As it starts to move out to the bed to start printing it should lower.
You should start by checking the screw in the coupler for the Z axis.
Mine have become loose a a few times.
After that calibrate the Z axis again.