I encountered a similar situation with a 12-Volt, 3-Amp SLA. It too had fallen below the minimum specified voltage. But after just two charge/discharge cycles the voltage came back to 12.6 Volts. It would appear that no permanent has been caused since the cell still holds its charge for approx. the same amount of time as previously. I thought that once the voltage had discharged to the critical level, then that would be the end of it. So I'm assuming I got lucky because the battery is less than a year old and otherwise reasonably healthy.
For anyone interested, as Martin rightly suggested, I first trickle-charged the cell from a standard car battery charger. After approx. 15 hours the voltage across the cell terminals had risen to 10.3 Volts. It was then discharged through a 12-volt, 21-watt lamp until the terminals were reading 5.5 volts. This was followed by 2 hours of charging at a rate of 5 Amps, followed by a further three hours of trickle charge (0.3 Amp).
It's been working okay since, but it's too early to tell what the long-term effect might be.