A question mark appears when the application couldn't be found. Have you checked if it's in the Trash or another folder?
If so, reinstall Mac OS X. The process to reinstall depends on the Mac OS X version you have.
If it is Snow Leopard or older, insert the Mac OS X disc, hold the C key while your Mac is starting up and follow the steps. After reinstalling, open > Software Update, and install all the updates.
If it is OS X Lion or Mountain Lion, hold Command and R keys while your Mac is starting up and reinstall OS X.
You won't lose anything reinstalling OS X, but it's better to make a backup with Time Machine > http://support.apple.com/kb/HT1427