a guy had this a month or two ago, in his case it was the air release valve in the pump, which sits under the drivers head light, look up under the bumper and you'll see it.
The other possible is the sensor, chassis to suspesion arm drivers side of dif, there is a basic guide in maintenance on testing it, if the sensor arm is slowly lifted in increments and the car hight responds and stops at a relative position to it then i would suggest the release valve at fault, stick another pump in it from a breakers on here if they have one.
If the car just tops out no matter what you do with the sensor arm i guess its a sensor issue. Money on the pump release valve though i think.
There is no reset as such afaik, but to get the car down unclip the air hoses on the shock body, look in between wheel arch and top the tyre, you'll see the air line and r clip there. Pulling the r clip allows the connecter to come off and release the air, you may well need to jack it up to support the car while you do this, and to refit.