If you noticed it immediately following an EQ, it may be that the cell was close to developing a short. Equalization has a tendency to produce a bit of heat, particularly if they are run for a long time, which can trigger a short if it is already close to forming anyway.
Does the battery still 'work' under a load when it is isolated (obviously at a lower voltage than rated)?
If so, you're probably looking at exactly that, a shorted cell. You might be able to 'cook' it out, as Tom suggested, but thats delicate, requires some insight (for lack of a better word) by watching meters closely and adjusting charge rates, insane amounts of patience, and can even be dangerous - If you manage to get it cooking and it is gassing, and the short decides to suddenly show back up... I really don't need to say what can happen next, do I?
If not, its an open cell, and theres absolutely nothing you can do about that.
Since your bank is not individual cells that you can manipulate by themselves by isolating them, there's little you can do with the damaged battery. Best thing at that point would be look into a new battery (and personally, I'd look into a new
bank, not just a single battery... but thats just me.)
Avoids the headaches of mismatch that will crop up on you in the near future due to differing ages and subsequently, capacities.
That decision will ultimately come down to how expensive that replacement is. If you have a smaller capacity bank, just fork it up and be done with it. You won't regret it later. If it's a sizeable (and therefore expensive) bank, weigh your pros and cons and do some tests to determine the health of the others before making your final decision. If one cell failed, others may be close to failure as well, but if they can be determined healthy, there may not be any point in replacing them yet.
Just remember, an RE system is only as good as it's battery bank! (as you can see...)
Just my $0.02
Steve