Sounds like he discharged it too much. If you have a volt meter, try to get the voltage of each cell using the balancing plug. If both are below 3.0V connect the pack to a standard Ni-CD/Ni-MH charger for just a few minutes. This should allow you to get the individual cell voltages up above the 3.0v mark. Once the pack is above 3v/cell you should be able to start charging it as normal.
Be sure to keep a close eye on the pack through it's next few cycles. Of course depending on the quality of the pack, and how badly it was over dis-charged, this may or may not work.
Just be very careful with it. It would also pay to have either a lipo sack or other fireproof container to put the battery in throughout this process.
Good luck. Keep us posted.