If the battery went completely dead; it has already lost the information stored in its memory; so disconnecting the battery will make no further difference.
Please bear in mind that a good battery can become completely discharged in less than a week; if one of the diodes in the alternator shorts out. This is not an uncommon thing to happen to an alternator. If the battery became discharged as a result of an alternator diode shorting out; it probably can be revived by removing the battery from the car and having it charged overnight with a battery charger of at least 6 amps charging capacity. Buying a new battery will waste money and not solve this problem in such a situation; as either a new battery or a battery that is recharged will quickly become drained, soon after it is connected to a car that has a defective alternator.
Here's what you can do to prevent wasting time and money by misdiagnosing this situation:
1> Buy a "computer memory saver" from an auto parts store. This is a device which contains a receptacle that mates with a 9 volt alkaline battery; and is designed to be plugged into the car's cigarette lighter. If there is any significant charge remaining in the car's battery; plugging this device into the cigarette lighter will enable the car's battery to then be removed; while preserving the data in the computer's memory. After a new or known good battery is installed in the car; the memory saver can then be unplugged. But this will only work if there was some power left in the car's battery at the time it was disconnected. Even enough power to light the dome light would be enough to save the memory data.
2> Remove the car battery and take it to the NAPA store to be recharged and then load tested. If the battery accepts the charge, and then passes a load test; it does not need to be replaced. But this most likely indicates that the battery did not become discharged due to age. And that means the car's alternator is likely to have a shorted diode. In that case; the battery should not be connected to the car; as it may quickly become discharged again. In that situation; the alternator should be replaced with one from the NAPA store, before the car is driven.
3> If the battery does not accept a charge; or fails the load test after being recharged; it should be replaced and reinstalled in the car. It would be good idea to have the alternator tested soon afterwards; but this is more of a precaution than an absolute requirement.