if the voltage gets too low on the batteries the charger will not turn on, it has to see enough voltage before it will even start charging.
They do this to prevent such a large amp draw on the charger to help prevent damaging it.
Each battery should read around 6v but no less than 5 (total volts of about 22 to 24v) before the charger will pick up and charge on it's own.
there are 2 ways you can get the batteries back to a suitable level so the onboard charger will startup again.
- get a small trickle charger and hook to each battery and get them charged back up enough so the charger will start working again.
-also you can get another 24v source and parallel it across the battery pack and fool the charger into thinking the voltage is good enough.
I have done the 2nd suggestion before but the load on the charger is high and it has a tendency to shut down early because of overheating so this may have to be done several times to get the batteries back charged completely.