Thanks a lot for your help.
I checked the install DVD since I can't see anything with my mac and found out it was OS X version 10.5.7, and I bought it in August 2009 at school apple store.
Also I did follow some of your recommendation, first I pressed the (left side) Shift-Control-Option keys and the power button at the same time and then released all the keys and the power button at the same time.
However, when I turned on again, it was same as before.
So, I pressed and held the Command-Option-P-R keys before the gray screen appears then released these keys after I heard the startup sound for the second time.
Still, nothing happened.
Please tell me next step might be useful or do I have to bring it to service center like the person above?
I suspect the reason might be because I did not shut down my mac for a long time, I only made it sleep. Is this the reason?
I really appreciate your help.