This is a laptop? An iBook G4 or a PowerBook G4 perhaps?

Take the OS X installation disk and try to boot from it.
Shut down the computer, restart and hold down the "C" key until you see the spinning gear.

Did it boot correctly?
If it will not boot from the disk, reboot and hold down the Option key until you see the Select Startup Disk prompt.

The Command-Option-P-R sequence is the correct way to zap the PRAM but all the keys must be held down at the same time. If only the Option key is held down, you will get the select startup disk prompt.

If you have been running the computer, then you do have the cables necessary to run the other, identical, one.
I guess you mean that you do not have two power adapters.

Check for dust bunnies at the air vents.

Let us know how you get on