I just saw that my answer to "Who can acknowledge my resignation letter?" has been "unaccepted". Given that it had been accepted for month I am surprised is there a justification for that ?
I've had this happen to me on Stack Overflow a few times. When it happens, I just leave a nice comment for the asker saying something like this:
Hey userXYZ, I just noticed the unaccept on the answer, and I thought I'd check in and see if you need any help. Can you elaborate on what the problem is in an edit to your question?
This tells them that I saw the unaccept and also demonstrates my willingness to provide whatever might now be missing from my answer.
In some cases, a user may unaccept an answer if something didn't work as planned, or if they wish to try and attract more answers. Just be constructive when reaching out, and everything should work out.
The usual reason for this is that another answer has been added, and the question asker thinks that answer is a better fit than yours. You might take this opportunity to improve your answer, or you might decide that the asker is right - the new answer does suit better.
It's pretty rare to see an unaccept when a new answer hasn't been added. The one time I saw it was on SO where someone went and unaccepted all the accepted answers they could as a way of protesting some site policy they disliked. It had nothing to do with the answer or the answerer and there was nothing anyone could do about it.
If you go look at the question and there is no accepted answer at all any more, just shrug and move on. If the now-accepted answer (whoever replaced you) deserves an upvote, give it one. I wouldn't leave a meta comment asking the reason for the switch, I'm not sure there's much to gain from that. If a reread of your old answer inspires you to improve it while you're there, go ahead - and then you could comment with an @mention for the asker saying what you've done. But especially on this site, where subjectivity and opinion play a bigger role than they do in, say, programming language syntax, askers changing their mind is just something that can happen and not really something you need to react to.