First, try to get into Recovery Mode. Remove the battery, then replace it and hold down both volume buttons and the power button until you see a menu. If even that doesn't work, for Samsung phones, the absolute last resort to get it into some kind of flashable state is to force it into Download Mode using what's called a
USB jig. You can make a jig yourself by connecting 301k ohms of resistors between two pins on a micro USB plug you won't miss or buy one from eBay. If that fails,
then ship it off for warranty.