Take it one step at a time. Check the bobbin holder, is it threaded right, is the bobbin in the right way? Re thread the machine, check that you have the same thread on the machine and on the bobbin. You can use different threads, but do this for now, to try and right the problem first.
Clean out any lint under needle plate, and around the bobbin holder if there is any, and a spot of oil is always useful. Check if there are any stray threads under the needle plate, or where the thread goes between tension discs. Best of luck.