Unanswered Z Axis off

When I try to home the printer (Robo 3D R1plus) it seems to be pushing down on the plate and in matter control is says the co-ordinates are X:0.00 Y:0.00 Z-0.80. There are no macros installed, I am running Matter Control 1.5. The printer was running fine for awhile and then we started having this problem. How can I reset the home position back to all 0's?