My subaru legacy won't start?

I replaced battery on my subaru legacy a few months ago. She has been running fine with no issues at all. I haven't driven in a few days. I went out this morning and she's completely dead. No lights/radio or anything left on. I tried to start her up and nothing. No dash lights, headlights and no turn over at all. I don't know anything about cars. What could the problem be?