To start off we should first determine whether or not your hard drive has really crashed or not.
If you can't boot up your Apple computer you first stop for diagnosis should be discussions.
apple.
com.
Do you get a gray screen, specific error messages or see unusual flickering? Whatever the particular symptoms you are experiencing, the first place to check is Apple's discussion page to see whether other users have experienced similar problems.
Often there are posts by other Apple owners that can detail simple solutions based on what you're experiencing.
Tips: make sure to select the proper category (Apple desktop, Powerbook, etc) and your model so you're sure to get accurate search results.
Also, try a few different variations on keywords when searching because people often describe things differently.
Next we want to find out if your drive will boot at all.
If you have another Macintosh around, then turn off the computer in question and plug it in to a working Mac with a firewire cable.
You can now power up the questionable drive, holding down the T key as your Mac tries to boot up.
If successful you'll see your (possibly) damaged drive on the desktop of the other computer.
Good time to do a backup of important files! Tips: After backing up key files, best to go back to the the discussions page again and do more searches because chances are very high that you don't have a damaged drive, but that something has become corrupted.
Once in a while, booting up the drive with a different mac will actually solve the problem so make sure you try to boot up the machine again.
(Don't forget to drag the drive to the trash, power down the computer and disconnect the cables!) If none of the above is working, then the best thing to do next is download some Macintosh data recovery software and see if the file system is just corrupted or if indeed there's a bigger problem.
Follow the links below for more details on what kind of software to get.
The process is simple -- you can download some free software, do a test to see if the data is recoverable, then purchase the software should you want to take the next step to back up all of your files.
If the data recovery software didn't work, then your next step is going to be sending the drive off to a specialist, because at this point, you've likely got a mechanical problem that can't be fixed with any off the shelf solutions.
Best of luck!