ChitownRC

I have a MRC Superbrain 989 and a TrueRc 7.4 2S2P 6400mah Li-Po.

I left the battery plugged in to my Mamba 6900kv system overnight by accident. Now the charger won't charge the battery (Equinox balancer wont discharge either). The charger says the battery voltage it too low to charge (~2.4V). I am assuming the worst... A month old $80 battery is gone... What do I do? Am I missing something?

this is what i was told to do. when i did that i was told to "jump" it with a regular charger on a low setting for about a min. it worked for me

That is about it.
You have to jump start that battery for it to be charged on the charger you use now.

Letting it drain down to low kills the battery.
As soon as you feel the power loss---that is the absolute min you want to take it to. It does not hurt to keep topping it off.