Gerard,
I Just would add some additional information,
I just re-read your original first post, and saw that you said, "I have open the firmware" , so the question is: what do you mean by this "I have open the firmware", ?
Did you do something "special" on your System before this problem ?
Is your Operating System installed in good conditions, of course you cannot boot from the installer CD, but what you can try at first is to launch a Permission Repair on your booting HD, and try to boot in single user mode in order to repair your HD if necessary.
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To run a Permission Repair follow these procedure below :
-Launch Disk Utility , located here: Applications / Utilities / Disk Utility
-select First Aid Tab
-Choose the Startup HD in the left window part
-Click on Permissions Repair button
-Quit DU
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Then boot in single user mode and launch the fsck procedure, to repair your booting drive. Do it by the following procedure :
-reboot while holding down both keys
Apple and
S
-You will get a screen full of text, which looks like the old MS-DOS
-at prompt :
-type
fsck -fy
-press
enter
-wait for the message resulting after the process
-if the message don't say that your
HD seems to be OK
-re type again
fsck -fy
-press
enter
-do it until you get a message saying that your
HD seems to be OK
-Then type
reboot
-Press
enter
be carefull when typing fsck -fy to respect space between
fsck and
-fy
Francis