A few hours ago this question How to Make a Large 々 With the Keyboard had several comments attached to the question, but now they are gone. Where did they go?
One of the reasons comments are deleted is that they're obsolete—either the question has been updated or an answer has been posted, and as a result the comments are no longer necessary. If a user flags a comment as obsolete, moderators look to see whether they're still necessary and, if not, they delete them.
I deleted these comments:
In this case, the question has been edited and now contains an image, so it's no longer unclear what the OP is asking about and we don't need comments asking for clarification. And the answer makes the remaining comments unnecessary—there is nothing special to type, and the "typing" portion of the question was based on a false assumption that the answer corrected.
For the general policy, see How do comments work?:
When should comments be deleted?
Comments are temporary "Post-It" notes left on a question or answer. You should not expect them to be around forever: Once a clarification has been made, an edit added to the post to include new information, or the issue in the comment is otherwise resolved, it is subject to deletion. In reality, many obsolete or chatty comments remain untouched due to the high volume of comments posted, but this does not mean that they can't or shouldn't be deleted in the future.
We leave more comments around on Japanese.SE than on some other SE sites because our comments often have useful information that's not contained anywhere else, but obsolete comments like these are still subject to deletion.
Please feel free to flag obsolete posts yourself so they can be deleted, and if you find something that should not have been deleted, feel free to explain why in a Meta post so it can be undeleted.