GoSew--if you could attach a picture showing top and bottom it would help us help you. But in meantime I would first open up the bobbin area-- it appears that your machine has a drop in bobbin. Use your manual and see how to remove the bobbin "case" and then use a brush (I use a cheap "chip" paint brush) and clean out any link--you might be surprised!. next reinsert the bobbin into the "case"--make sure you get it lined up right and have the thread coming off the bobbin correctly. then re-thread the top thread--have the presser foot UP when you do this so the thread seats in the tensioner disks. Make sure you needle is inserted correctly and all the way up. Like Sewingpup says, if the top thread just lays on top it's a bobbin tension issue- while drop-in bobbins supposily are not adjustable, they actually are, you just have to figure out where the tension screw is and sometimes it's hard/impossible to get to--which means a trip to repair shop. If it almost looks good on top (little loose) but the bobbin thread is not "locked" or a "bird's nest" its the top tension. All adjustments need to be tiny increments and do make note of where you start. If this doesn't help it might be a timing issue (did you hit a pin?, even sometimes where many seams come together can be a problem) and that means a trip to the repair shop. Hope this helps!