When you get really frustrated it's a good time to go do something else for a while. I have a Pfaff machiine which I dearly love but sometimes there is a knot or a lump in the thread and it will kick it out of the tension disks. If you take a hold of the thread above the machine and tug gently on the thread behind the needle it should put the thread back in the tension disks. If that doesn't work, it pays to rethread, or go have a cup of tea (or a glass of wine!).