I have a similar problem with S5.I have sent a board to Bitmain.They will check.I have never over clocked.

I have measured the voltage of working and not working board.I have found the problematic chip by this way.If a chip is malfunctioning the board does not work.

I recommend you to measure the voltage of first and last chips.

Today, I can not access to the miner.I am suspicious of a virus which effects miners.