I've not been a P3 owner for long so I'm still quite new to this platform in terms of familiarity. Turned it on last night, wouldn't boot, immediately start basic troubleshooting. I see everything in the cabinet including the TV is getting power but nothing in the backbox is except for the amp. Start following wire paths, OK, everything past and including the backbox ATX power supply is dead, so it must be something related to that. Do the usual re-seating, look for frayed wires, take voltage readings, etc., nothing.
Oh yeah, aren't they supposed to have real good support documentation online? I go to the MM website and the support section, search "power," and on page 2 is this gem: https://www.multimorphic.com/support/projects/customer-support/wiki/Powerup_Troubleshooting
Followed all the steps starting with #2 since I clearly had line-in power, hit that motherboard reset button, ah ha, everything now has power so it's probably the CMOS battery. Take out the graphics card, replace the battery, reboot, follow the very detailed steps to put the BIOS back to factory defaults, and I'm up and playing again in 20 minutes!
Brilliant! Just a really well-written page of steps and solutions that brought back a non-booting pin in a flash. I have no idea if every P3 problem will have such detailed documentation, but man, that was smooth. Kudos.