MacBook keeps showing question mark on startup
Sounds like your hard disk could be on its way out.

A folder icon with a question mark means that the Mac can not find a valid Operating System on the disk (or the OS which was on there has become corrupted).

Booting with your finger on shift boots the Mac into, what is called, Safe Mode (or Safe Boot). That is to say no extra software is loaded at boot time, only the essentials, as well as some checks being performed. Quoting the Apple web site for Safe Mode:


  Starting up in Safe Mode does several things:
  
  
  It verifies your startup disk, and attempts to repair directory    issues if needed.
  Only required kernel extensions are loaded.
  All user installed fonts are disabled while you are in Safe Mode.
  Startup Items and Login Items are not opened during startup and login    on OS X v10.4 or later.
  In OS X 10.4 and later, font caches that are stored in    /Library/Caches/com.apple.ATS/uid/ are moved to the Trash (where uid
  is a user ID number).
  In OS X v10.3.9 or earlier, Safe Mode opens only Apple-installed    startup items. These items are usually located in
  /Library/StartupItems. These items are different from user-selected
  account login items.
  
  
  Together, these changes can help resolve or isolate certain issues on
  your startup disk.


Your best bet is to start your Mac using an install OS X CD (or USB stick) or an external disk drive containing another installation of Mac OS X, and run Disk Utility (or preferably Disk Doctor) from that and/or reinstall the OS. Better still, maybe seriously consider replacing the hard disk, if it is getting old, and failing.

Also, it maybe worth copying off any data that you require (or do not  wish to lose).  However, saying that, you can always use your [time machine] backup to recover any lost files. You have been making regular backups, haven't you?