Do you have a retail 10.6 install DVD? Put your install DVD into the optical drive (CD/DVD drive) and reboot. As soon as you hear the boot chime, hold down the "c" key on your keyboard (or the Option Key until the Install Disk shows up) until the apple shows up. That will force your MacBook to boot from the install DVD in the optical drive.
Or if you are running 10.7 Lion, boot from the recovery partition (Command +R on boot) and use Disk Utility to repair your OS 10.7 partition.
When it does start up, you'll see a panel asking you to choose your language. Choose your language and press the Return key on your keyboard once. It will then present you with an Installation window. Completely ignore this window and click on Utilities in the top menu and scroll down to Disk Utility and click it. When it comes up is your Hard Drive in the list on the left?
If it is, then click on the Mac OS partition of your hard drive in the left hand list. Then select the First Aid Tab and run Repair Disk. The Repair Disk button won't be available until you've clicked on the Mac OS partition on your hard drive. If that repairs any problems run it again until the green OK appears and then run Repair Permissions. After repairing use Startup Disk from the same menu to choose your hard drive for restarting from your hard drive.
If your hard drive isn’t recognized in Disk Utility then your hard drive is probably dead.
