After replacing the HDD and the battery my Mac Mini G4 no longer boots, there's no chime or any image output. So far I tried to reset the PMU by pressing the button on the motherboard without power, but it didn't help. I also tried pressing and holding the power button for 10 seconds, I got a beep but nothing changed. If I remove the RAM stick I also get a shorter beep.

Is there anything else I could try?

You can try PRAM reset:
Power On & Key Combo: Press the power button, then immediately press and hold these four keys simultaneously: Option (⌥) + Command (⌘) + P + R.

D'oh, I should have thought of that! Thanks, that fixed it. After holding Windows + Alt + P + R (PC keyboard) eventually I heard the boot chime.