What I am trying to do is get you to boot from the installation disk, that way the disk maybe can be repaired so that it will boot up normally.
If the installation disk is not in the DVD drive then insert it now.
Then press the power button until the machine shuts completely off.
Press and hold down the letter C and press power to start the machine, keep the letter C pressed until the Apple logo and spinning gear appear, then lot go.
Read the following instructions very carefully please. They fill in details of what I just told you.
To boot from the installation disc and the Mac is on then start at step #1.
If your Mac is off or will not boot from the hard drive then goto step #3.
1. Insert disc #1, it can be the original that came with your Mac or it can be a retail version newer than the original that came with the Mac.
2. If the disc opens then click on the Install OS X icon, if it does not automatically open then double click the disk icon on the desk top and then click Install OS X.
Your Mac will restart using the install disk.
3. If your Mac is on but will not boot up then insert the installation disk and turn it off by pressing and holding the power until it shuts down.
Now press the power to start it and hold the 'Option' key until the startup manager appears.
If your Mac is off then press the power to start it up, immediately press and hold the 'Option' key for the startup manager.
Insert the disc and click the circle with the arrow to refresh and then select the installation disc to boot from.
Click continue at the language screen, the next screen will be welcome and at the very top of the screen will find a menu bar item Utilities.
The countdown menu select Disk Utility.
Select your hard drive from the list on the left, probably Mac HD, then click on First Aid as shown below.
Run Disk Repair, do not use verify, then run Permissions Repair, again do not verify.
After this finishes quit Disk Utility and the installer to restart, or power off using the power button.
Now restart and see what happens.
If the above fails the a new installation will be necessary, if done correctly none of your data such as movies, music, photos etc will be lost.
Let me know.