@spaceman:
The most important thing to check is that the Power LED (green one) is dimming up after about half a second. If it isn't, Exec (kernel) hasn't been started. Turn off your machine immediately if this isn't happening! A "Caps Lock" light that isn't responding after 10 times, is probably indicating the same, as the keyboard can buffer 10 (or thereabout) inputs before it needs to pass them over to the system.
You should try booting your system with another PSU. I had a problem similar to yours on an A4000T. The problem was partially solved. Thread:
dead A4000T. This machine would sometimes start Exec, and sometimes not. That was because the main board didn't get enough power (bad connection).
