Apple has a very well written article on what to do when you cannot startup your Mac running OS X (10.x) involving using the tools provided on your System Installtion/Restore CD or DVD.
The article can be found in their support sectiong at http://support.apple.com/kb/HT2956
The tools allow you to check the hard drive for errors that could be corrupting your system files. Damaged or corrupted files critical to the Mac OS could prevent the system from starting up normally. A hard drive can develop faults due to physical damage (hard knocks and bumps), normal wear and tear from everyday use, viruses or a combination of the above.
Should a physical test of the hard drive fail to find any faults, you may need to reinstall your Mac OS files. The install/restore disc will compare the installed files with the originals and replace them as necessary.
Please read the full Apple article before attempting a system restore and remember to always make backups of your work.
Starting up from the CD or DVD
Usually, you can start from the disc by putting it in your computer, restarting, and holding the C key. Or, put it in the computer and click the Install or Restore icon you see in the disc’s main window (after which the computer will start from the disc without you needing to hold C.)
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