the pack fell below the 6v(3v per cell) safety limit set by the lipo charge mode. You must set the charger in nimh mode 6 cell at .3 amp.
Charge the pack until it reads more than 6 v....You must watch this step because if the pack gets more then 8.4v you will cause a fire.....
Once the pack is above the 6 volts stop the charge and reset the charger for the lipo pack and charge like normal.