If the switch is bad it can be repaired, and rather easily at that. My PFO ballast had a problem several years ago where one lamp would flicker and go out on occasion. I tried several bulbs in there and the problem persisted. I finally broke out the multimeter and found that the switch was faulty due to high resistance. It hadn't opened completely but apparently had carbon buildup on the contacts from years of arcing when turning off or on or had some sort of internal fault. Whatever the case, I got a new switch at Home Depot (pick your favorite store), soldered it in, and haven't had any trouble in years.
As for your corals, I wouldn't be too worried. Take the ballast out of service and either get it repaired or replaced within a few days to a week and they won't notice a thing. To them it will be like having storm clouds block the sun. You certainly don't want to go for an extended period without lights but short periods of time won't hurt and some folks will even argue that it can be beneficial.
Mike