Astra 55 plate 1.9 SRi CDTi - Fob wont lock the car!!!???

Hi I am rather foxed by this. When I got out of the car the other day, as I took the key out of the ignition, straightaway the warning noise came on (bing, bing, bing, bing etc). This only happens if for say I have left the lights on etc. The radio/CD normally turns off automatically when I turn off the ignition, but this was still on. I then shut the door and tried to lock the car with the fob. It did nothing. So I opened the door and straightaway I got the warning noise. The car has been unlocked now for two days. The only door I can lock is the driver's door and this is by manually locking it by putting the key in the door. Trouble is, this doesnt lock the whole car, only the driver's door. My initial thought was the battery in the fob, but then i tried the spare key fob which i never use and got the same response i.e. it wont lock the car. Does anyone know of anything I can do? I am female and dont know anything about cars!!! Simple fool proof solutions would be gratefully received :-)