Explain to me why I can put away a sewing machine in excellent working order, and then when I pull it out again it insists that the tension is wrong wrong wrong. I am not threading it any differently than I ever did. Now to go unearth the manual.
In my experience, it is the act of attempting to fix it that solves the problem. It's a sort of propitiation of the sewing gods. Nothing actually gets fixed, the swing machine just forgives you.