How can I fix my central locking on my Vectra (2004)?

I've recently encountered a problem with the central locking on my vectra. I tried to lock it and the doors locks came on and then re-opened. When I get in and start the engine, the LED displays the icon for the drivers door being open when it's actually shut and, therefore, the interior light won't go off and I can't turn on the alarm or lock the car properly without doing it all manually. Any ideas??