If nothing that you're aware of brought this on, when you last shut it down, if it still had some charge left but you put it on charge, my guess is that a power surge down the mains (perhaps associated with a thunderstorm) may have fried it. You haven't stated the condition under which it failed - was it that it stopped charging ie the battery had become flat so it wouldn't power up and connecting chargers made no difference or it simply stopped working as in was working OK but then while you were using it, the screen went off and nothing you've tried will resurrect it. 'Stopped charging/working' doesnt give any leads as to whats going on.
If its just where you can take it in order to have it looked at professionally, Samsung have stores located in many cities within many countries. The Samsung website for your country will tell you where your nearest store is. I've never needed to have a repair undertaken but in my experience they did offer excellent training about my product at both of my 'local' stores.
