If you cannot get at least 12.6 V into the battery then it needs to be replaced. That would be the first thing to do. When battery voltage drops too low on most vehicles the system will automatically turn off the headlights, reduce interior lighting, etc. in order to save power from the battery. That's what happened when my alternator went out
The low voltage being detected could also not allow certain switches and sensors to work. A sensor reads when the brake pedal is depressed so that you can only shift gearing when on the brakes. This prevents people from shifting out of D accidently and such
How bad was the head-on? It could have possibly damaged or un-screwed some crucial ground wiring which might prevent the starter from turning. Is the engine even turning over, or does it just make a "whirring" sound, etc.? There are a LOT of different things that could be wrong.
Hope this helps some......