You are in something that is called a boot loop. There are options to recover from this but your phone may be toast now. One method I know to get out of a boot loop, is to reboot into recovery mode.
Just turn off your phone completely and then press and hole the power button, vol. down, and the home button all at the same time. Hold this combination until you see the boot logo. Now just use your volume keys to highlight the cache partition option, and use your power key to select the option. Now once you reboot, you should be out of this boot loop.
If this does not work, im sure someone else in the forums can help you out.