You said "s board", is this an S model machine or an S+ machine?
Did you check fuses? I think there are 3 fuses in the lower part of machine, 24vac, 8vac and 7vac or something like that. Maybe the 8 volt ac fuse is blown.
Also, did you inspect the display board carefully? It may have a burned part. Have you added any new bulbs recently, like some LED's maybe?
If the fuses are ok and the display board doesn't have any damage then pull out your motherboard and look it over. The motherboard is the small board in the bottom of the machine that the mpu board inserts into.