I wasn't aware that the new Daystate batteries are Lipo and not nickel–metal hydride battery anymore. My bad.
If that's the case, the lipo battery is 8.4v and called a 2S in RC hobby, meaning two 3.7V cells in series. Each cell is rated at 3.7V, but has a charging curve of 3v-4.2v. If the battery's voltage falls below 3V the battery's internal circuit disables it to prevent it from damage. It also disables charging if the voltage drops much below 3v....like 2.8V and below.
The lipo requires special lipo charger, cannot use a regular nmih or LiFe battery charger. The 2s lipo need a balanced charger which charges each cell separately and evenly. That's why it has 3 prongs, one for ground and two for voltage for each cells. The charger cuts off at 4.2V charge for each cell.
You should still be able to test your rifle by using a 9V battery briefly, because the circuit board must have a voltage regulator down to 5V (or 3.3V) which are the normal voltages for microcontroller electronic circuits.
So your charger may be bad. Or you charger connections going to the battery are lose. Or the battery went way below 2.8V and the charger won't kick in. Either way to do any troubleshooting you must be able to read the voltage on the battery pins. And go from there.
If the lipo is dead, the "dead" lipo can be recharged by raising the voltage in it to at least 3V level so the charger will "recognize" it again and allows charging. One way to do that is to take 3V-3.3V from another battery to each cell and slowly build back up the voltage to above 3V inside. Do not use much higher voltages or it could damage the battery.
