I had a problem once and they had me loosen my tension all the way, as loose as you could get it. Then take a piece of heavy thread and run it through the tension wheel on the inside of the machine. You can do that without taking anything apart. Just hold the ends of the heavy thread and pull back and forth. There may be lint or thread lodged in there and you need to get it out.
Then put your pressure foot in the down position and turn your tension wheel to about 4. Rethread both the machine and bobbin and try again.
Hope this helps.
Barb