06 VW Golf 1.9 TDi won't start?

My golf is usually pretty reliable. Its done 135k miles and had a new clutch and ignition barrel in August this year. I've tried to unlock the boot tonight using the central locking key and it will unlock the doors but not the boot When I try to lock the car, it will only lock the drivers door and front passenger door (I've had to manually lock the back passenger doors). Even worse, the car will not turn over when I put the key in the ignition. The interior lights come on but they're much dimmer than normal. All the lights on the dashboard come on as normal but there's no life in her at all, she doesn't make a sound when I try to turn the engine over. Any idea's what the problem could be? The lights on the dashboard seem to indicate that it could be the alternator but I'm aware from reading the manual that this light only goes off when the car ignites. I'm getting married next year and I'm really hoping that this isn't an expensive problem to fix.