It's stuck in a 'boot loop'. You can try booting into 'safe mode' first and clearing cache, that usually works when things go bonkers. (With the device off, press and hold the Power, Home, and Volume Up buttons until your phone boots). If this doesn't work, you might have to do a factory reset, but this would mean losing all of your info and saved data
You can also try connecting your phone to Kies via USB and let it sync.