Is that before or after rebooting your device?
A little more detail would help. From what you have said here, I take it that it says that "The application has stopped responding."
If you have access to a computer, try backing up all your essential stuff on the device using Samsung Kies before continuing so as to prevent the loss of any data you might need (assuming that the device is on and standing by).
What you can try doing now is clearing the cache memory via recovery mode.
This is done by turning the phone off then holding Power, Volume Up and Home buttons at the same time until the device powers on. When it turns on, release the Power button but continue holding the other 2 buttons until you see the recovery menu.
From there, using the volume keys for navigation, scroll down to the Clear Cache option and select it using the home button. After this action occurs, you should be back at the initial menu, then you select the Reboot option.
Let me know how it goes.