White screen and flashing folder with (?) question mark on it

The flashing question mark indicates that the startup disk could not be found or that you're holding down the mouse or trackpad button. If you see the flashing question mark and then it goes ahead and starts up then all you need to do is set the startup disk in the Startup Disk preferences. If your computer is not starting up then you should follow the steps under the heading "Steps for Mac OS X" in this link: [A flashing question mark appears when you start your Mac|http://support.apple.com/kb/TS1440].

Most likely, one of those steps in the link will solve the problem. If not, you might have directory damage that's too severe for Disk Utility (mentioned in the link) to handle or you might even have a mechanically dead hard disk.