Sounds like the alternator or the battery went south? Might even be a short or something else? Sorry, I know it's not much help. Check that the alternator belt is still there, check all your grounds, including the one going to the battery and if possible, drive it to an auto parts store and have them hook up the battery testing job. My girlfriends 2.5GT Legacy had the battery light come on and they tested it and said the alternator was going south before it failed.

Does the car have a cold air intake? Does any of the electronics still work. If it won't start, will the lights still turn on, radio still play? Sorry, just trying to brainstorm some ideas. I wish I could help you more.