If the cell reads millivolts, wow, then its absolutely and totally gone. No hope of reviving that. When a cell is discharged below 2.5 or 3V, most chargers will refuse to charge it. You can usually "revive" those cells by using a "smart" charger (4 button charger) and setting it to NiMH, and charge that way until you get above 3V, and then charge like a lipo again. If you do that, your cell will have been damaged, and you will have lost performance, but it may still be usable IF the discharge wasnt too extreme and IF it wasnt in this state for too long (think minutes, maybe hours, but not days). Ive "revived" batteries that where discharged to 1.5V per cell that way, but anything lower than that seems hopeless.As for continuing to use it as a 2 cell pack; you probably can if you absolutely have to, but be very careful. Never charge it unattended and assume it can catch fire at any moment. lipo fires are no joke. Just get a new one, and dispose of this battery safely (fully discharge it in salt water or make some fire works by driving a nail through it in a place where its safe the thing to burn/smoke/explode).
So Ultimately I can only get 8.4V of power from that battery. I also observed battery has puffed up little bit.
Sounds damaged.... especially since you're getting well below the usual voltage.... and puffy battery. Time for a new one. Also helps to have spares.... and maybe need to have some kind of current monitoring or current limiting system to avoid drawing too much current from the battery.When it comes to rc and batteries.... have spares on hand. And check on power handling specs to try cut down on issues. Sometimes the issues can be due to quality issues. Hard to say what happened right now.