In reply to 5cifi:
I'd always recommend iron-on seam seal tape (see CurlyStevo's post), but you may not even need that. First, try taking a cool-warm iron over the seam, protecting iron and garment with a piece of brown paper between the two. You may find that you can re-melt the original adhesive into place again.
If not, use the iron technique to remove all the loose stuff before you apply new tape.