If you purchased a MacBook in 2006 or 2007:
Apple has determined that a very small percentage of hard drives that were used in MacBook systems, sold between approximately May 2006 and December 2007, may fail under certain conditions. If your MacBook was purchased in the date range listed above and shows a flashing question mark on the screen, please take it to Apple or an Apple Authorized Service Provider and they will examine the hard drive to confirm if it is eligible for a replacement, free of charge, under this program.
More details here: