more than likely your bobbin is jammed and your machine is dirty take a small screw driver and an old tooth brush and take the bed plate off the bobbin case and bobbin out and clean the hook bobbin case and feed dogs even take the feed dogs off of the feed dog arm and make sure there isn't any thread reminants under that flip the machine over and clean it as well

after this clean and oil the machine thru out and see if it solves the issue also the little tension spring on the bobbin case can have stuff trapped under it take it off and clean the bobbincase