There could be many reasons for this problem. But very often the Mac needs to have the PRAM (Parameter Ram contains settings for the clock, and other internal features) reset. Doing this cannot hurt the machine. Here are the steps:
1. Shut down your Mac.
2. Locate the following keys on the keyboard: Command (⌘), Option, P, and R. You will need to hold these keys down simultaneously in step 4.
3. Turn on the computer.
4. Press and hold the Command-Option-P-R keys before the gray screen appears.
5. Hold the keys down until the computer restarts and you hear the startup sound for the second time.
6. Release the keys.
If the problem was a corrupt PRAM your computer should start.
If this doesn't solve the problem try putting yout original Operating DVD in and booting from it.
If you still have problems it is most likely a hardware problem. I always look at the most likely hardware to fail and that is the hard drive. There are good instructions on the internet on how to remove the hard drive for your make and model. Do it yourself or find a friend that is comfortable doing this or take it to a local computer repair store or of course Apple will fix it.
Molly's computer had a bad hard drive and as soon as I disabled the drive I was able to boot to the OS DVD. I replaced the hard drive and re-installed the Operating system. This is a good time to mention backups. If you have done your backups you won't lose data, if not you are out of luck.
Here a some links that will help with PRAM