If the computer was running Mac OS X 10.6.8 or earlier, insert a compatible install DVD and restart with the C key held down.
If the computer was running Mac OS X 10.7 or 10.8, press the Command and R keys.
If the hard disk appears in the Disk Utility, run the Repair Disk command on it. If the issue persists, reinstall the OS.
If it doesn't appear, it needs to be plugged back in or replaced.
Here is a support article that may assist you:
More often than not, there is a problem with the HDD. If it cannot be repaired in Disk Utility, you may have to replace it.
My regards to the previous posters, but lets try a easy fix first.
When you get the Flashing Question Mark on a folder when booting, that means the firmware cannot find the bootable operating system the firmware is assigned to boot from.
It could easily be the firmware forgot what it's supposed to boot from, or it could be worst like a corrupted OS X or dead drive.
So the easy test is to do this
1: Reboot the machine holding the option key down on the built-in keyboard.
A list of icons should appear, one is OS X and the others may be BootCamp or Recovery HD depending on your OS X verison (you never stated it) or if you installed Windows or not.
If you see the OS X one, then click on it's arrow to boot it and in System Preferences set the Startup Disk again.
If that fixes it, then your done.
If not, then your going to have to run through this list of fixes here, they get harder for harder problems further on the list, but if it's a software issue it should be fixed, if it's a hardware issue then Apple will have be called in.
Thanks all. Did try to reboot holding down option key and computer couldn't find the start up disk. Ended up bringing in to Mac Specialist where they confirmed my hard drive was dead. Needed to replace it and lost some things that weren't backed up. Really frustrating since my laptop was only a little over a year old.
Remember to back up your stuff, people!