You can try this, restart your computer holding down the mouse button. To do this properly the second you hear the startup sound, hold it down and do not let go-if the computer goes as far as the desktop then it didn't work.
Using the paper clip method can be a little difficult, but if you use a flashlight you can see through the holes on the G5 and guide your paperclip point into the manual eject hole and push in-it should have tension on it, if not you have your paperclip angled to much and it slid passed the metal it should of pushed against. If that is too difficult you can remove your CD drive so you can see better.
