I don't have a Janome, but this has happened to me on my Pfaff. Look in your manual for installing the quilting guide or darning foot and you will probably see that screw. On all the machines I've used over the years there is a thumb screw that inserts at a downward angle (impossible to see the hole) to keep the quilting guide or darning foot in place. It is easy to accidentally unscrew the screw just rubbing fabric against it repeatedly, or it can vibrate loose to the point it will fall out. After I do any FMQ I make sure I re-tighten the screw in the hole so I don't have this problem.