Does the LED in the switch illuminate when on, if not then you need to be checking the supply to the switch, if it does then i would be checking the connections to the screen itself, you should hear a change in the engine sound when the screen is on due the load it has on the electrical system?

As to the key could it simply be the battery that needs replacing, if not and I am not sure by any means but perhaps the ariel is around the screen area and it got knocked etc?

Martin