Could be simple as the battery. If its old it may have enough power for simple jobs but not enough amps to start the car. If you are a member of a motoring organisation get them to come out and test it. if not try jump starting the car from one with a good battery, if its starts you have found your problem, them take the battery to a good garage or factors and let them test it just to be certain before buying another. Also check the battery connections and vehicle earths.