Why did he suddenly stop talking to me?!?

He probably does like you and the fact you're denying liking him to everyone is probably getting him down. If you're talking together so much you probably do have chemistry so just keep acting like normal and it will soon pass. However, if you like him it's best to tell him soon or else he may give up on you and accept you don't like him back. Hope this helps and by the way, being on MSN so much really isn't healthy - talk to him in the real world.....might help.