That happens when the pressure foot is not down or the thread has come out of one of the loopers or tension on your machine..... if it is on the bottom of fabic it is the top trension... try rethreading your machine and if your going real fast the thread is looping around something it should not have... Mine does that when the thread loops around the tension because I am sewing tooo fast for the machine....try that and see if it helps.... loopy on top is your bobbin thread