There must be a sewing machine virus going around! I had this exact thing happen to me on Tuesday night - the thud, the loose thread on the back, etc....I also was very tired when it happened, actually cried (and I rarely cry), vowed to give up "stupid" sewing forever....went back into the sewing room on Wed night, did basically everything that everyone here has mentioned and now my machine is back to it's old self....sewing right along...