Perhaps it came loose itself with use . I tightened it and got back to work . When I switched to my straight stitch machine , again I found the foot had been unscrewed and was loose . While I do occasionally need to switch feet on this machine , I had done no sewing the day before that would have forced me to unscrew the foot to switch it out .

Question: How many machines do I have ?
Answer: The one with the loose foot and the straight stich machine .Four machines all with loose foot .None of the above choices .Too many to count them .