Z azis build plate stuck at the bottom on mbot grid 11 +

The build plate is stuck at the bottom , cant seem to get it to move, home doesnt work or the jog mode.It seems as though there is power going to it but it may be jammed? I wondered if anyone else has had a similar problem, any suggestions greatly appreciated.

solved! it was a piece of extruded abs that has managed to get stuck in the limit switch on the extruder !!