The question mark means the computer is not finding a volume that it can boot off of. If you can boot to the OS install disk check disk utility and see if A. the drive is shown and B. attempt to repair the volume if it is showing up here. Provided the drive shows a repair disk could get you up and running. If the repair fails though it is a reformat and reinstall of the OS.
If the drive is not shown than you want to reset the SMC/PRAM and than see if the drive is shown afterwards.
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