Ooo - that sounds so familiar.
If there's only the one buttonhole you're not happy with, I'd have a think about undoing and knitting the whole toot again.
You could try this:-
Look closely at the loose thread and if it seems long enough, then snip it in the middle and put each end to the back - inside - of the garment. If you're handy with a needle, secure each end at the back with small but firm stitches. If you're using an Aran weight wool, it should be possible to secure each end with yarn that you've spliced into two from the ball.
It's also possible to use fabric glue to secure the two halves of the loose thread but I prefer to stitch them down.
Hope this helps.