Have you done anything to try and fix it? Have you rooted your phone, or were you trying to root, or do any other mod at the time?
If you haven't already, start with a battery pull. Leave the battery out for a few minutes, then put it back in and try to power up.
If that doesn't work, try booting into recovery:
Make sure the phone is off. Pull the battery if you're not sure. With the phone off, press, all at the same time, Volume UP, home and power. Hold all three buttons until you see the word Samsung, then release all three buttons. Note that if you release too soon, or too late, the phone will skip recovery and try to boot normally. If this happens, just power down and try again.
If you can get to recovery, you should see a menu with several options. Use the volume buttons to move the selection to Clear Cache, then press power to execute that selection. After it's complete, try to reboot.
If that doesn't work, you probably need to try a factory reset. Do the same as above, but instead of choosing Clear Cache, select Factory Reset, then press power. The phone will reboot, and hopefully boot normally. You'll need to reinstall all your apps, and ll your app settings, and any data that's not on the SD card, and not synced with Google or some other cloud service will be lost.