I bought a new battery for my car yesterday and today it is dead, what could it be?
Alternator likely is not charging the battery as it should. If you can get a jump start from a friend or neighbor, have the shop that replaced the battery test the alternator.
Otherwise it could be a bad electrical connection or fuse problem. Any “Check Engine” lights on?
Dang, I was afraid of that! 
It could also be a defective battery. I bought a battery that didn’t make it through a week-end. When I measured the voltage on the battery, it was about 9 volts. I charged it up overnight with the battery cables disconnected. I had it charged to 12.6 volts. I left the battery cables disconnected and measured the voltage 2 hours later. The voltage had dropped back to 9 volts.
The store where I bought the battery didn’t want to exchange it. The clerk wanted to tell me that the problem was in the car. After an argument, I hooked up his load tester and showed him it was bad. He then said that the battery I had purchased was out of stock and I would have to buy a more expensive battery. At this point, I asked for my money back. He countered that since they had recycled my old battery, he couldn’t do that. Finally, the manager showed up. They gave me a defunct battery and my money back. I went down the street and bought a battery someplace else. The sad thing is that I had bought a lot of merchandise from this store–Quality Farm and Fleet. Maybe this kind of attitude on the part of the employees is why the chain no longer exists.
i hope that it is just that and not the alternator, that would be alot cheaper! And thank you both for answering my question so quick!
Did you buy the battery and installed it yourself? Or, was it purchased at a repair shop and installed by them? What was the reason behind replacing the battery?
It could be your old battery was fine, but there could be corrosion on the battery terminals. If you put in a new battery yourself without cleaning the battery terminal connections you will have a battery that “seems” dead because current can’t flow properly through the corroded contacts.
Hubby went to start the car and nothing turned on. Got a boost and then took the battery to a auto parts store and they checked the voltage and it was nearly none. Got a new one and they replaced it. Brought it home, and drove it around and then this morning…dead, again. I will tell hubby to check coils! Thanks! 
The other, worse, possibility here is that you have a parasitic drain on the electrical system that is draining the battery while the engine is off. The reason that I say this is worse is that the drain could be very difficult to correctly diagnose down to which electronic module or whatever else is to blame.
Oye vey! I pray that it is something simple to diagnose! Thanks for the info that is good to know!