Sudden shutdown, runs, then shutdown again, now won't post

Looking here for any suggestions to troubleshoot.

Standard NAS Killer 4.0 build with an X9SCM and E3-1270 Xeon. Less concerned with describing the individual components of my build and looking for suggestions on where to start based on my symptoms. Build thread is here: Migrated from 2U 4.0 Killer to 4U

Server has been running great, but randomly shutdown at 0300 2 nights ago. It was not a controlled shutdown. I booted it back up fine, parity check completed (Unraid) and ran for a couple days until it shutdown on its own this afternoon.

Front power button doesn’t work now.

Won’t boot, won’t post. I cycled the PSU power switch and that initiates a power cycle. CPU fan doesn’t turn but all case fans turn for a second, then stop. Green lights turn on the mobo and front power button still doesn’t work. This on/off fan cycle repeats indefinitely until I turn off the PSU at the switch. All the green mobo lights turn, network indicator light turns on, along with my USB card. Front power indicator light does not turn on.

Looking where to start based on these symptoms.