All added comments have disappeared!
Are you really talking - as tagged - about Writer (where the function is called Insert Comment) or are you talking about “add a comment” function of this site (If so please retag “meta” and remove tag writer).
Insert>Comment allows you to annotate your Writer document in such a way it does not print (unless you explicitly request it).
Comments provide a means to keep notes about the progress of the work, record caveats or new ideas, … outside the main flow. In this way, you still have a final view on the document while out-of-band information does not disturb the “discourse”.
Comments can disappear only if you select Delete All Comments from the drop-down menu in any comment.
To show the community your question has been answered, click the ✓ next to the correct answer, and “upvote” by clicking on the ^ arrow of any helpful answers. These are the mechanisms for communicating the quality of the Q&A on this site. Thanks!
In case you need clarification, edit your question (not an answer which is reserved for solutions) or comment the relevant answer.
PS: considering the wording “add a comment” in your question, I suspect it might be related to the AskLO site. If this is the case, be more explicit and edit your question to improve it. Also, all questions about this site must be tagged meta. Retag if needed.
Comments can disappear only if you select Delete All Comments from the drop-down menu in any comment.
… or when saving to e.g. plain text 
- and sometimes it’s accidental, like tdf#123476.