Q: wont start

This will reinstall OS X without erasing your drive. Please be sure you backed up.

1. Restart the computer. Immediately after the chime hold down the COMMAND and R keys until the Utility Menu appears.
2. Select Disk Utility and click on the Continue button.
3. Select the indented (usually, Macintosh HD) volume entry from the the side list.
4. Click on the First Aid tab in the toolbar. Wait until the Done button activates, then click on it.
5. Quit Disk Utility and return to the Utility Menu.
6. Select Reinstall OS X and click on the Continue button.

Also, see this User Tip: Basic Steps for Upgrading OS X.

If you are using an earlier version of OS X then follow these instructions:

Make sure you have a current backup before proceeding.

1. Restart the computer and after the chime press and hold down the COMMAND and R keys until the Utility Menu appears.
2. Choose Disk Utility from the Utility Menu and click on the Continue button. Repair the Hard Drive and Permissions as follows.
3. After Disk Utility loads select the indented (usually, Macintosh HD) entry from the the side list.  Click on the First Aid tab, then click on the Repair Disk button. If Disk Utility reports any errors that have been fixed, then re-run Repair Disk until no errors are reported. If no errors are reported click on the Repair Permissions button.
4. When the process finishes, quit Disk Utility and return to the Utility Menu.
5. Select Reinstall OS X and click on the Continue button.

My guess, then, is that the drive has failed. If you have a bootable external drive, then see if that will startup the computer. If that does not work, then it may be some other hardware problem.

Are you able to startup from the Recovery HD that is required for each of the methods I posted earlier? If so, then use Disk Utility to repair the drive. If it can't or if your drive does not even appear in the listing, then it is bad.