You don't give any information about the history of the device, so we can't guess at the possible cause of this - anything from a failed update to water damage could produce these symptoms. If you could post again with more information we might be able to give better advice.
Otherwise, three things you could try (in order):
* Pull the battery out and leave it out for a couple of minutes. See whether it will boot normally after that. This should clear any stuck bits in the RAM, which could be the cause.
* If that doesn't work, shut it down again then reboot while pressing both power and "volume down" and keep pressing both until the word "Samsung" appears. When it does, release power but keep the volume down key pressed. If you get a message saying that the device is in safe mode release the volume button and press OK.
Safe mode disables user-installed apps, so if one of your apps is the problem this should let you boot. Then you can try to work out what the problem is and fix it.
* If none of that makes any difference, try recovery mode. Shut the phone down then boot up again while pressing power, "volume up" and home buttons, and keep holding these until it enters the recovery program. From there you can wipe cache and dalvik cache, then see whether it will boot normally. If not, the last resort is to enter recovery again and do a factory reset. This last will wipe your device, which is why I say to try everything else first - none of the previous steps will cause data loss. Remove your SD card first: I don't think it should wipe that, but I don't own a Samsung so cannot be certain, so best to play safe. If you have any sort of problem with corrupted apps or data a factory reset will fix it.
Note that if your system software itself is corrupted, or if there is a hardware fault, then even the factory reset will not fix it. So there are no guarantees for any of the steps above, they are just the "generic" things I can think of to try in the absence of any further information about why the phone is in this state.
