Last night when I got home, I forgot to turn off my lights.

So I come out this morning and the battery is dead, so I go to Auto-Zone and get a loaner so that I can get my old battery charged. Well when I put in battery the car wont start. It doesn't even try to turn over, it just make a loud "clicking" sound.

What is the problem????. I need to find out soon as I have to leave in about an hour. Any insight would be greatly appreciated.