As long as there's no damage (swelling) you can try to rescue it by charging it as a NiMh. Use a low voltage like 0.3 - 0.5amps so the voltage doesn't increase too rapidly. If this is a 3S, charge it this was until voltage is back up over 9v (if its a 2S, then charge til voltage is over 6v). Stop the charge and then switch to a normal lipo charge and it shouild charge normally.

If at anytime, the lipo starts to swell, stop charging immediately and take the lipo somewhere where it can't do any damage if it goes into a runaway condition. Watch the lipo carefully and monitor all the charging. If it doesn't swell up, its prob going to be OK, but you should watch it for the next few cycles to be on the safe side.

Due to safety issues, lipo mode charging is set up to not charge lipo if the cell voltage is less than about 3v/cell. You're using the NiMh mode to get it back up over this minimum.