Where is the lvc on the Sidewinder set? You do know you're not supposed to
discharge a lipo below 3volts/cell (6v for a 2S)?? You shouldn't drive a lipo until it just quits unless there is a low voltage cutoff (lvc) to protect the lipo from being overdischarged.

If you charger is not 'seeing' it, you've prob overdischarged it and its below 6v. Lipo chargers are set to not charge any lipo thats below minimum voltage due to safety issues.