What is the battery voltage with the ignition on?

With the ignition on and the headlamps on, turn the key to start the car. Do the headlamps dull or go out or no change in brightness?

Sounds to me initially (without seeing the car or having more information) that the alternator may have given up or the alternator warning light was defective and that it may simply have run out of power.

What fuel is in it, i.e. how much?