Autohoming issues on z-axis

[zy400737]

I have just put a glass bed on my printer and use the bl-touch to help confirm the bed is level.

However, for some reason, when I went to Auto Home the printer the z-axis went upwards and not towards the bed. What ever I try to do, all the printer does is raise the z-axis, to the point it is approx. 20 cm of the bed and will not move downwards. The screen shows the z-axis at 10.5 cm what ever the height of the z-axis.

I have tried to reset the printer, turning it off and on, but nothing. Please can someone help.