BleepingComputer.com: Macbook Pro Cannot Boot/startup

Need your help with a MacBook Pro. Purchased this computer less than a year ago. Last fall, computer would not boot, and Apple Store diagnosed as faulty hard drive. They replaced it.

Today, tried to get back on computer after screensaver was on. The little beachball icon kept spinning, so I held power button for 5 sec, and it shut down. Upon restarting by pressing the on button, it went to a plain white screen about 15 seconds, then the gray folder with the question mark inside it just kept flashing. Can't get the computer to do anything now.

Please help if you can. Thanks.

A flashing question mark appears when you start your Mac

http://docs.info.apple.com/article.html?artnum=58042