Mine had these same issues. It wouldn't power, and it would just start to power on, but power off.
I took jumper J25 and set it so that the CMOS settings would reset. Upon doing this, and performing the typical power on/power off, moving J25 back in its original position, and then powering it on, it was working correctly (once I set the BIOS settings back).
Bizarre that it made the entire system not boot.
Yes, PSU = Power Supply Unit.
I replace Power Supply same thing Black screen No Post No BIOS no beeps just flashing light on from for a second then all stop then trys to restart again.
Sun Ultra 24 Workstation.
Next Step? Take out all video cards see if boots,
take out ram chips see if boots,
Thing has a battery in it someplace see if it is dead I guess?