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.