Did you leave your lights on yesterday after driving in the rain? The 'battery light indicator' you mention is probably the standard battery charge lamp which warns you if the alternator stops charging. Along with the oil pressure and handbrake lamps they illuminate when the ignition is switched on but go out the moment the engine is running (except the handbrake light of course).
I think you left your lights on yesterday and the battery is flat. Have you got a charger? If so, then disconnect the battery and charge it for a few hours. If not, then get someone with a set of jump leads to jump start the car or buy some leads yourself which you will then have for just this situation should it arise again. If you are with a recovery firm such as the AA or RAC they will come out and jump start the car.
However it starts, once the car is running, take it for a good run for an hour or so - but don't switch the engine off during that time. That will put a good charge back in the battery and it should then be fine.