Normally, a problem on the top means an issue with the bobbin. Did you check the bobbin? Did the thread jump out of the bobbin tension spring, or maybe get wound around the spindle in the case? If you use prewounds, sometimes they go wonky half way thru. If the bobbin was partially used, I'd try another bobbin. Also, if the bobbin was working all right for the first part, I would not mess with the bobbin tension unless you tried everything else.