hi hannah, presumably when you opened the control box cover there would have been a tamper warning which you would have had to reset? did low battery show immediately, if so you would have had to enter your code to stop the internal sounder. has the keypad simply shown low battery since then? The display will not return to normal of its own accord, you need to acknowledge the problem and then reset the keypad. there is no change to any settings required.
Assuming the battery is connected properly polarity wise and the battery fuse is intact then after 24 hours or so of a standby charge all should be well.
How are you checking the battery?
I would check that you have correct voltage at the battery leads first, reconnect battery fully then reset
the keypad. If internal sounder immediately sounds and indicates low battery then you have either a connection problem or a duff battery!