In this case, resetting the PRAM is likely not an option, as the system will not power up at all. The PRAM reset could certainly be done once the system boots.
The G4 tower is easy to open.
Just to clarify here, the PMU is a chip on the logic board that controls a number of items that manage power (hence the name, Power Management Unit). This unit is ALWAYS running, if the backup battery is good. As a result of an unexpected power cut (and other possible reasons) the PMU can stop, or lock up, preventing any start of the logic board, so the system can't boot. This can usually be restarted by pressing the reset button which usually can be found near the battery. Don't hold the button, just briefly press and release, that should be all it needs. Older systems (pre-G4) used a different chip to achieve something similar, and the reset button should be held down for several seconds, but not on newer systems. In fact, the G5 models now have an SMU (System Management Unit), different again, but still controls power, and other functions.
