First thing, what is the serial number of the unit?
And has it lost ground control ability also?
If the platform is stuck in the air you can manually lower it using the manual lowering **** most likely located in hydraulic motor compartment on a small manifold block near the tank.
If once you lower the platform the unit starts working again the problem may be in the control cable in the scissor assy or associated with the limit switches on the lifting mechanism. If control does not return then the problem is elsewhere.

To operate from the ground:
Turn the ground control key switch to the ground position (if it has a ground control box with a key) and then use the lift/lower button that is either beside the keyswitch on the base control.
Or if there is no key on the base unit it may just be a switch located on the panel inside the hydraulic motor compartment on the flip access cover.
If the ground lift/lower switch works and raises it past the 4ft mark then concentrate your attention to the cable in the scissor assy and control box in the platform control.

If you need any information on your unit you can go to skyjack website and get the service and parts book there in the support section free of charge, their catalog's are open information for everyone, just make sure you have the serial number of your unit handy so you can insure you get the correct information.