Lipo batteries have a minimum working voltage , drop below this voltage and the charger will register a fault by flashing red sometime green together,and sometime no light at all,
this means is the battery has been run to low for the balance charger ,
you can retrieve the battery by back feeding up the output wires for a few minutes, but this should only be attempted by a electrical technician only,
a battery left flat will damage the battery (sometimes shown as swelling ) and you will need to replace it.
Some model have warnings, buzzer or light flashing on the esc, no good if your model has a body cover on it,or its 100 foot up in the sky!
vigilance is the word, as soon as the edge goes off the performance, stop and recharge immediately.