If you have an original Power Mac G5 and are using the Apple Pro Keyboard (2003) that came with it, you may experience an occasional issue where your keyboard may not respond to your input. You might find that you can't enter text, or that a device connected to the keyboard's USB port, such as a mouse, doesn't work.
If this happens to you, unplug the keyboard from the computer and then plug it in again. This should make it work as expected. To prevent this from happening again, make sure that you have the keyboard connected directly to the computer—not through a USB hub or display. Also make sure that you've updated the computer's system software to Mac OS X 10.2.8 or later.