(portions from Apple support)
I've seen this work numerous times. Try this…
if you can start up from any cd that will load the Finder. (Disconnect everything except for keyboard and mouse. )
Note: If your hard disk is not available (mounted) when started up from CD, reset the PRAM as described below, then repeat these steps. If your hard disk is still not available after resetting PRAM, contact an Apple Authorized Service Provider or Apple Technical Support.
If you can't choose your hard disk in the Startup Disk control panel, your System Folder may have been altered or removed. In some cases, you may solve the issue by moving files back into the correct location in a process known as "reblessing" the System Folder. A "blessed" System Folder is one that is recognized as a valid startup System Folder, as indicated by the appearance of the Mac OS logo badge on the System Folder.
Reblessing the System Folder only works when all the original files are still installed and in the correct location (or you can put them back in the correct location). If any files have been deleted, you may need to reinstall system software. If you have deliberately removed System Folder files, this includes putting them back where you found them.
Additionally, you should only have one folder named "System Folder" on your disk (not to be confused with the folder named simply "System" that is installed by Mac OS X, if both systems are installed on the same disk).
To rebless your System Folder:
1. Start up from CD.
2. Close any open windows.
3. Open the hard disk.
4. Choose By Name from the View menu.
5. Locate the System Folder. Tip: You should only have one System Folder. If necessary, rename any other "System Folder" you're not using. Don't rename "System" if Mac OS X is also installed.
6. Open the System Folder. At this time, you should replace any items that you have previously removed from the System Folder, if applicable.
7. Do one of the following:
▪ If Mac OS X is not installed or is installed on a different volume than Mac OS 9, drag the System suitcase and the Finder to your hard disk icon. If you have more than one hard disk, drag the files to the hard disk where the System Folder is.
▪ If Mac OS X is installed on the same volume or partition as Mac OS 9, create a new folder outside of the System Folder but on the same disk, then drag the System suitcase and the Finder to the new folder.
8. Close the System Folder.
9. Drag the System suitcase and Finder back onto the System Folder icon.
When the Mac OS logo reappears over the System Folder, it has been reblessed. Select the folder in the Startup Disk control panel, then restart the computer.
Reset parameter RAM (PRAM)
If after resetting PRAM the computer starts up normally, reselect the startup disk in the Startup Disk control panel.
Hope this helps…Martin