Diagnosis
No power to the handset means a break in the low-voltage chain, usually a loose cable between the control box and the handset.
How to Fix It
Unplug the desk. The small, thin cable that runs from the control box to the handset has connectors at both ends. Trace it. Disconnect and firmly reconnect the end that plugs into the control box (it's a small rectangular connector). Do the same where it plugs into the underside of the handset mount. Ensure it's snapped in securely. Plug the desk back in.
If That Didn't Work
That handset cable could be damaged or faulty. If re-seating didn't help, you'll need a replacement cable. Contact Fully support; it's a standard part they can ship out.
Heads Up
Before you contact support, double-check the obvious: you're using the correct power brick (24V for Jarvis) and it's firmly plugged into a live outlet. Try a lamp in the same outlet to be absolutely sure.