Flashing folder with question mark at start up

I have a macbook pro that is about 3 years old. The other day, it completely froze for several minutes, so I just held the power button and turned it off. When I turned it back on, there was a gray screen with a flashing folder in the middle and a question mark inside the folder.

When I try to boot from the OS cd that I have, there is no volume that I can format my computer to....it's simply not seeing the hard drive. Does anyone have any ideas? Thanks.

Flashing Folder & "?" Mark at startup

Same just happened to me. I read through OSX manual and low and behold found the answer. Unplug (if using external power) and wait 10 minutes. Disconnect any peripherals. Press power-on button and immediately hold down command, option, "P" and "R" keys (resetting the PRam). You should hear a chiming sound. Keep holding keys down and let it chime a total of 3 times - then let go of the keys. It worked for me and all started up right away and system is working faster than it had been for months. One last thing, when start up complete go to your "Systems Preferences" then to "Startup disk" and re-select your hard drive to start up from it. Hope this helps and thanks for reading - writing this really helped my way-to-frazzled nerves comedown after my experience.
Take care all Mac-Addicts!

There is a sticky note about this subject guys.