The basic rule of thumb is this:
- Move on.
Beyond that it's really case by case. Some examples:
- If you left, for example, a snarky comment that still contained some useful info and it was removed, you could re-post the useful info sans snark.
- If the comment conversation didn't directly pertain to the post, then perhaps you could meet up with a user in chat to continue your discussion.
- If it was an unintended consequence of some system action, you could repost the comment.
- If the comments were moved to chat by a mod you have to respect that.
- If your comment was completely legit but in a sea of non-constructive comments and a mod just wiped them all to clean it up: Oh well. I suppose you could write it again, but if it gets removed again, respect that.
- If your comment was just non-constructive, then there is nothing you can or should do about this except learn from the experience.
Some things you should not do:
- Do not get into some sort of back-and-forth battle in comments about deleted comments.
- Do not repeatedly repost a deleted comment.
- Do not storm into some chat room and start an argument about deleted comments.
Those kinds of things not only waste your time and a moderator's time, but can also lead to more serious consequences for you.
One day, I made a comment that is useful. A moderator deleted it without any mention. So I had a chat with the moderator. He said "I'm so sorry. I didn't mean to delete~." It was his mistake.
This was a real human (presumably) volunteer in real life sitting at a computer who made an honest mistake while dealing with 100's or 1000's of other flags, comments, and complaints, and even took the time to apologize for it. This is the opposite of a problem. So you could:
- Take a moment to handle it yourself and repost the comment, or
- Politely ask that moderator if they wouldn't mind undeleting it, keeping in mind that they are not obligated to take the time to do this at all and that you must accept that possibility gracefully. Do not press the point or continue to pester if the answer is "no" or if no answer is given, or
- Do nothing.
I think we need something to observe moderators.
We have the Stack Exchange community managers who, unlike moderators, are employees of Stack Exchange keeping an eye on things. There are also the other moderators on a site keeping an eye on each other. And there is due process to potentially remove moderators, a process that we as a community were given the opportunity to discuss and contribute to before it was put in place.
But for the most part comments are transient beings, at any given time they could go away. So unless your comment was some critical detail that was directly related to the post being commented on and added good information to it (if it would make for a good answer / edit on its own you can also take that route), I would just let it go.