For questions about comments that were deleted, deleted comments in general, or the comment deletion process

In general:

Who can delete comments?

A user may delete one of their own comments at any time by clicking on the (x) icon that appears to the right of the comment's timestamp when the comment is moused over.

Comments that are flagged by multiple users are deleted automatically. The number of flags needed is usually based on the comment's score. It currently takes 3 + (Score / 3) flags (rounded up) to delete a comment. Comments containing certain keywords can be deleted with a single flag.

Moderators can delete any comment, or purge all comments from a post. They also have the ability to move all of a post's comments to chat.

See also How do comments work? and Reduce the threshold for comment deletion with flags