There might be a lever inside that the stitch length knob hits to make your stitches longer & shorter,what I think happened is when he turned it all the way out it dropped down,so what you need to do is,turn it all the way out,look in the back access plate & push it in reverse so you see what's moving & take a screwdriver & hold it up while you turn the adjuster back in.You'll probably need to hold the lever down as you turn the knob back in too.