This has happened to me too... and I found extra pieces of thread I miased.. and the machine won't work until you get every piece of thread out... also have a look at the way you have thread the machine... sometimes you might have missed something... so rethread the machine as well... also before you start sewing hold on to the threads at the back of the work, as they can jumble up underneath for some reason... this is what I do anyway, hope it helps you in the future.