If the default launcher (TouchWiz) becomes corrupted, you may need to perform a factory data reset to get functionality back. This will remove user data and downloaded apps but should leave your files on the sd card/partition alone.
If you still have the phone try this:
- Turn the power off. Pull the battery out and reinsert it to make sure.
- Hold the Volume Down button.
- Press and hold the Power button until the device turns on (the screen may flash once or twice).
- You are now presented with the Recovery menu.
- Highlight Clear Storage by pressing the Volume Down button.
- Press and release the Power button to select.
- Now confirm by pressing Volume Up for YES and Volume Down for NO
Once complete, the phone will reboot.