If you have the installation discs that came with computer when you bought it, then see what happens when you boot from the installer Disc One.
Booting From An OS X Installer Disc
1. Insert OS X Installer Disc into the optical drive.
2. Restart the computer.
3. Immediately after the chime press and hold down the "C" key.
4. Release the key when the spinning gear below the dark gray Apple
5. Wait for installer to finish loading.
I'm opened my Mac this morning and got past the user login! Strangely its came on all on its own. Now I'm using to type this message. Although the problem could well still exist and its something I can't afford to leave alone.
Why do you think this has happened and does this help determine the hardware/software issue?