Check the sensor bar at the bottom of the mast for impact damage - if the sensor bar is to far away from the sensor on the mast carriage the truck wont be able to tell where carriage is.

System works like this:
Lift the forks from the floor, sensor gets to the end of the bar & it then knows where it is & the calibrated height value.
At this point the height counter on the carriage takes over.

Do you have a BT CAN service key?

If you do you can bring up the diagnostic screens & pin point exactly which of the sensors or height counter pulse channels is missing & causing the problem.