I would bet money that the battery is toast. It might hold a charge for a few hours. Might even hold one for 8-10 hours if you're lucky, but if it has sat for a while, and discharged, then just consider it toast.
Spent the 75 bucks for a new, cheap walmart battery and be done with it.

If the battery was good, there are only so many things you can leave on in the car to kill it over night. your headlights.. your stereo, at a decent volume, a big fat amp, Or maybe a combination of every possible light you have.
If it is something dumb, like a glove box light, or even the dome light, the battery might not have enough nuts left to start the car, but you'll still have some power. You're saying you have Zero power. So either your definition of "over night" is more like 18 plus hours, or you powers of description really blow or, the battery is toast.

There really isn't any good way to test for a bad cell. Autozone's machine might tell you that it has a bad cell or whatever, but I wouldn't bet money on it being accurate. I've had them test batteries that didn't have any fluid in them just for shits and they said they were perfect batteries.

EDIT: look, here's the deal. a battery has a bunch of cells in it. For the sake of arguement, and keeping the math more simple, we'll say there are 4 cells. Ok, so when it was new, it had 400 cranking amps. After a period of time, those cells start to age and drop in power. We'll say it is something like 10% per year. So say the battery is 2 years old. that means that the thing can only put out a max of 320 cranking amps. Now, you add in the fact that it has been sitting for a while, which increases the aging effects. Take it a step further and now account for the dead cell. So instead of 4 cells with 80 amps each, making 320, you only have 3, with a total of 240 amps.
Because of the way a battery works, it tries to balance out each cell. So the 3 good cells are constantly sharing power with the dead cell trying to balance it out. The problem is, the dead cell can't store power, it just discharges. So you get 4 cells, with 60, the dead discharges, leaving 180. it averages out again, so each gets 45. it discharges. leaving a total of 135.. etc etc etc etc. so you can see how a battery can hold a charge for a little while, but won't have a charge come morning.