[Raion]

So I own an Onyx2, and here's my startup procedure from a graphics console:

1. Flip the breaker on the back, wait for the SMC to light up. You MUST have cables attached including keyboard and the graphics cable.
2. Turn the key to On, and all 9 fans should start up
3. The monitor will show the IR3s booting up, but mine takes nearly 15 mins to come up due ostensibly to the fact I have two RM10s and a rather heavy setup of 4G of RAM, 4x400MHz CPUs. YMMV.

For a serial connection, if I recall, it's standard 9600 baud to the console port next to the breaker and that will show you the entirety of the boot process.

I think you need to connect either over serial to see, or try connecting a keyboard, it would say on the SMC if something was the matter likely.