One of the cells in one of my 4s, 10AH batteries was down to 2.7 volts. That's bad; I usually run them down to 3.2, maybe 3.0 exceptionally. And when I tried to recharge it, it wouldn't recharge.
I put it aside, and replaced it with a pair of 4s 5AH in parallel, but I didn't stop thinking about it. In the last week or so, I've got it back into action. What I did was balance the cells (which means discharging the cells that aren't the lowest voltage) and then charging, then balancing, then charging, and after a few iterations of this, I've got it back to 4.2 volts.
I'll use it next time I go out, to see if it really is fixed.