You need to restart your computer. Hold down the power button for several seconds...
Restart while holding x; if this works, go to System Preferences, click Startup Disk and ensure the correct disk is selected. Or, restart holding down Option, releasing it when icons appear; you'll be asked which disk to start up from. Or zap the PRAM (see the tip above).
If none of those work, perhaps you've moved, renamed or deleted a system file. Reversing this may sort it out – or not.
The last resort is to reinstall Mac OS X. Start up from your Mac's OS X DVD, run the installer and choose Archive and Install. You'll need to reinstall your apps, though.