Welcome to the forum, Gael. I moved your topic to the Sims Medieval Forum.
You didn't mention what quest your blacksmith was on - was it the same quest both times, or different ones? I haven't had this particular problem but I did have a knight character that seemed to be bugged - the screen would go completely black when I played him. I got rid of that character, made a new knight, and haven't had the problem again. Who knows why some of these things happen!