iMac doesn't bong, the fan spins

I recently got myself an iMac G4, when I first plugged it in and pressed the power button, the DVD drive made a noise and the fan spun up, it spun until I turned it off since it didn’t even display any video. I tried reseting the PMU, reseating the RAM (and replacing the internal stick), I tried to boot it without the modem daughter board, I also installed a new PRAM battery, all to no changes. What can I do to get it working?