you don't say--is your's a drop in bobbin or a side? if it uses a separate bobbin case, make sure that you are having it unroll in the case counter-clockwise and that thread is seating in the tension arm of the bobbin. It looks like your quilting was fine? did you have your presser feet dropped? think back to settings and what you were doing when it was still working fine. Then start doing some back tracking--but as others have said, bottom thread nests are almost always a top tension issue--is the thread a cone or a spool? Is it coming off the cone/spool to it's going straight to the next tensioner? Check the check spring that is part of the tensioner ensemble on most domestic machines--sometimes they get worn and while they don't break, they don't spring either! also make sure that the thread is seated in the tensioner. Good luck, nothing is ore exasperating than a machine not working!