Make sure the white rectangular connector at the end of the ribbon cable has not worked its way loose from the control unit. The control unit is held to the machine by 4 small screws. Remove these and carefully & slowly lift the control unit away from its mounting until you can see the ribbon connector. Push it home firmly & try again.
If the unit is powering up its display from the batteries, but without response when the machine is used, it would suggest the display is OK, but there is no input from the machine's sensors - which will be via the ribbon cable.
Phew..! - hope this helps...
Jerry
February 2011