Bottom line is, if you haven't rooted the phone or run non-stock firmware on it, don't risk doing that now to try and fix it. If it borks, you'll end up voiding your warranty.
Send it into Samsung/an authorised repairer & have it repaired under warranty is my advice (contact Samsung in your country to get details of repairer).
Now, if you have rooted the phone/run non-stock firmware on it, and you can get into download mode then you can still fix it. If you can't get into download mode tho, there's no chance of you fixing it so don't even attempt to. Send it to a service centre & hope they don't notice you've been messing with it.