what exactly did you do? step by step would be helpful. Can you get it into download mode? (Do this by holding the down volume and power at the same time. if you get it into download mode, you will see a large yellow triangle on the screen with an android holding a shovel). If so, then it's recoverable.
Also, were you trying to put a custom rom on there, or was this due to an ota update or software download from samsung?