Why?

I've just played a 10 min game which I was winning and then the opponent doesn't move and his/her time runs out. Anyway, when the time runs out nothing shows up to say I've won.(The opponent's clock was down to 00.00) So I logged off for a while and when I log back on later it says in my Games list that I lost the game because I resigned! I can understand if you log off whilst the game is going on that that would count as a resignation but it was over.

Does anyone know why?

Your computer and internet connection bugged.  It happens.  For best results, clean your cache daily.

i've had this problem also lately.

http://www.chess.com/forum/view/help-support/game-abandoned

good luck getting a response about it. i'm still waiting.