What else is on and what else doesn't work? Are the gauge cluster lights dim? More than likely, the battery is just too weak. Check the fusible link also in the fuse panel under the hood for the alternator. If that is blown, the alternator won't charge the battery and of course the battery will die. Check your alternator connections along with the tension of it. You might not have unhooked the alternator but there is a chance that it might have gotten unhooked by some odd instance.