if above did not work here's another thing to try: boot the machine into Open Firmware by doing the following:
1. restart the machine, and immediately hold down these 4 keys simultaneously: Cmd-Option-O-F (that's a letter O, not a numeral zero before the F)
the machine should boot but show a white screen instead of the normal Mac OS
2. at the prompt, type: eject cd then press Return
this should hopefully eject the disc and show OK on the screen.
3. at the prompt, type: mac-boot then press Return
your machine should boot normally