My MacBook Pro has a flashing question mark in a folder against a grey background
Either the Mac OS X installation is corrupt and won't startup the computer or your hard drive is faulty, physically.

You could try to connect another Mac to your mac via Firewire or Thunderbolt in Target Disk mode to actually see something. 
That's what the Target disk mode is made for.

You could also try booting from the original DVD/USB, then run Disk Utility. If you can see the hard drive in the list, try repairing the disk.