Power Mac G4 won't boot after installing

[seni]

I’m trying to update my Power Mac G4 (AGP graphics, 512 Mb, 500 MHZ) to OS X Panther and now the computer doesn’t restart.
I updated to OS 9.2.2, and after installing the firmware to my model – 4.2.8, the power button on the tower blinks, alternating between green and orange, the monitor becomes grey and the Mac won’t boot. First I thought that was normal and I should wait a while, but two hours and a half latter the Mac still didn’t boot. When I press the shutdown button I heard a long tone, the same happens to the restart button. Any ideas? I really need to install OS X, but more then that I need my Mac back to life.