Problem with G30 code for Z axes

Hello guys,

at school we have a printer Prusa I3 without LCD that looks exactly like the one shown in this tutorial : [data.emotion-tech.com]

One day, a schoolmate lifted it when it was printing, then (may be he unplugged and plugged back something without telling me) but the nozzle was digging the plate and never stopping it. So i decided (too quickly) to load back the firmware but i loaded a way to recent one so it mades things worst.

I finally succed to upload the right firmware and get all my axes working, the only problem i find is i can't set up correctly the Z probe calibration, the led prob is working fine.. following the tutorial, im using G92 Z100, then moving the nozzle towards a paper, then G92 Z0, then lifting the Z axe up tp 30 mm.. at this time, they ask to command G30 but the Z axes is going up and never going towards the plate..

I thought it could be a connection problem but that didn't fix it.

if someone have a clue it would be really helpful

cheers
gui