After driving my 945 home from work with nothing out of the ordinary, the next morning I came out to find the battery completely flat. No dashboard lights, no cranking of the engine or clicking of the starter, no dome lights, nothing.

I am assuming that either (1) something was running that drained the battery, like the aftermarket CD changer the PO installed in the car, or a dome light (although nothing was left on that I can remember), (2) there was a sudden malfunction in the electrical system (alternator, relay, or something like that), or (3) the battery is toast.

I am completely ignorant regarding electrical systems, so somebody please help me out. What should I look for after I get the car started, either by charging the battery or jumping it?

Thanks.