Have a voltage reference from whatever on hand, compared to a voltage divider, so that the voltage divider is just a tad lower than the reference at the cut off voltage, then, using the comparator, drive a FET to switch everything off/on have all the circuitry on the switched side of the FET, then use a momentary tact switch to manually drive the gate of the FET to turn it back on, also have a resistor to stop the gate from floating.
When the battery goes flat, the comparator will turn the FET off, and the circuit will stay off because the comparator isn't getting any power, nor is the voltage divider, it won't oscillate, then once you recharge the battery, you press the tact switch to turn the FET on, it should power the detection circuitry and keep the FET on until it goes flat again.