This just happened to me, miner was hashing at 9.5 TH/s and one of the small red LED light on the board was not lit.

 The troubleshooting sequence was

1. Unplug all sockets from PSU to S9 and replug them in making sure they are tight.
2. Try another PSU
3. Reset the S9 and re-load firmware

None of it worked. So had to create a repair ticket and sent it to Cali for repairs. The complete unit for S9 needs to be returned for warranty repairs. Bitmain also stated that it will take a week to repair.

Warranty steps were pretty straightforward.

Hopefully I will get it back soon.!