My desk is showing a CON error message on the handset and won't respond to commands.

The "CON" error message stands for Connection. It means that the connection between the cables and the parts (Handset, control box, motor) is faulty, a part is not working, or you may need to unplug and replug the cables to reestablish the connection. 

This happens when the Handset or Controller does not get a clear connection with the control box. It can be that the control box is not receiving current from the motors or legs or a part is not working right sending the message "CON" to the handset. 

 The best way to address this is by unplugging all connections and cables, cleaning the ports, then reconnecting all cables. Make sure that the cables are secured and anything that would connect to the control box will have a "clicking sound" to know its secured. After reconnecting all the cables and connectors, recalibrate the desk. 

       Here is the link to the re-calibration Manual: Recalibration Steps and Procedures

       Here is a video on how to re-calibrate your desk: Recalibrating your desk

Once fixed, it is best to recalibrate your desk once every 2 weeks to make sure that the legs are aligned as the flow of the current can be uneven or would reach one leg motor faster than the other.