Either the switch is broken or the unit itself has a fault. I'd advise you to take your car into the garage or dealership and see what they can do for you.
When you try to turn it on does it click? If not the compressor is not engaging. A low level of refrigerant will keep the compressor from coming on and is the most common problem. If it is low, you need to check for a leak.
the ac system is probably broken. When the car notices that the coolant pressure is to low it will not allow the ac to work to prevent damage to the compressor. You probably have a leak in the system.
there is few things this could be. if the system is low in pressure the lo pressure switch will disable the compressor. also these vans also have a common failure in the clutch solenoid
It could be a number of things:
-The refrigerant is low and needs to be recharged.
-The low pressure switch is stuck open.
-The AC relay could be stuck open.
-Blown fuse.
-Bad compressor clutch.
Take it to a professional for proper diagnosis.
check is there any leakage in the AC system.If there is any leakage in tube sealed out.Then engine turbine sensor is not working or wire the sensor has a broken wire.