/** I am not responsible for your phone. Many options listed here will void your warranty and/or irreparably damage your phone! Proceed with caution and at your own risk! **/
Hi there! As was mentioned, this is known as a boot-loop. This is usually a recoverable state, but you need to be EXTREMELY CAREFUL if you choose some of the more daunting items on this list; many WILL brick your phone for good (unrecoverable state). You don't give a ton of context as to your experience level, so this list is a little drawn out, but, without further ado, here it is:
USE YOUR WARRANTY. If you haven't already voided it and your phone is still under warranty, use it! All other options on this list will void it. DO THIS FIRST!
RECOVER FROM A BACKUP. If you got into this state by dabbling lightly into the world of roots and custom roms, it's likely you already have installed a custom recovery. If that's the case, and you followed instructions to a T, almost every guide will have had you create a backup. Booting into recovery (hold volume up+home+power from a cold start) will give you the option to restore your backup, or minimally the boot image from it.*
GO NUCLEAR. Essentially the same idea as the recovery mode, but you'll need a computer. You'll want to flash the "stock rom" or "stock firmware" here. IT IS SUPREMELY IMPORTANT that you find the CORRECT VERSION. If you don't, your phone is likely forfeit. Kiss it goodbye. A website like Sammobile keeps a directory of stock firmwares to pick from. Again, CHOOSE THE RIGHT ONE or your phone goes kaput. The basic idea is to use a program called ODIN to flash the stock rom (or minimally the boot image from it) to your phone in download mode (hold volume down+home+power from a cold start). There are many guides online for this, so I won't go step by step here, but Google something to the effect of "Flash stock rom using ODIN" and you'll get some good results.
Hope this works out for you; lemme know if you need anything more!
*Having a custom recovery with a backup is wonderfully handy. Once any android phone of mine is out of warranty (even unrooted ones!) I always flash a custom recovery and perform/keep a backup. I'm a pretty heavy power user and often find myself in boot-loops or the like. Being able to flash a fresh boot image straight from my phone's recovery has saved me many a trip to my computer, not to mention my sanity :D