What did you do before that happened? What I suggest you to do now is do a hard reset. To perform a factory reset simply follow these steps:
1. Turn device completely off. (May need to pull battery for a moment)
2. Hold down the Volume Up and Power keys.
3. Release the Power key when you see the Samsung screen, but continue to hold down the Volume Up key.
4. Release the Volume Up key once you see the Android System Recovery screen.
5. Use the Volume Down key to hilight Wipe Data/Factory Reset.
6. Press the Power key to select it.
7. Use the Volume Down key to hilight Yes -- delete all user data.
8. Press the Power key to select it.
9. Press the Power key once again to select Reboot System Now.
Let us know what happen. Good luck.
I did these exact steps and my phone is now working! thanks you so much I didn't know what to do with out my cell :P