Is the edge of the fabric frayed or does it just look like the thread is missing? I had a quilt top with a gap in one seam and I thought, what the heck? Then I remembered I had pulled a thread that was in the seam sticking up on the top of the quilt when I pressed the top. Apparently that thread tail I yanked pulled out the bobbin thread on the seam. If it is just the thread missing, use the invisible ladder stitch and sew it back together by hand.