Mine has one of those little plastic drop in bobbin cases. For some reason the tension suddenly vanished. No matter what I did I couldn't get it to hold tension. I ordered a new one and after I cleaned and oiled my machine I limped it along for a while. Then no matter what I tried the thread kept breaking. I finally had to give up and take it to the shop.
There was a whole bunch of thread caught in a place I couldn't see and the timing was goofed up.
It's sewing like a dream now.