Odd. I don't know the answer, but a couple of things that are worth trying:
1. Boot into recovery*, select Clear Cache with the volume buttons, ten press the power button. Then reboot. If that doesn't work,
2. Go to the Application manager, tap the menu button, and select Reset App Preferences.
*Power the phone completely off. Simultaneously press and hold the volume UP, home and power buttons. When you see the Samsung logo (not before) release the buttons. When the phone is done booting you'll see a menu you can navigate up and down with the volume keys, and execute with the power button.