You say nothing about the forum you refer to and whether you are the owner of the forum.
If you are the owner, it is possible that parts of your forum have remained unvisited. Posts that are not viewed do become scheduled for deletion after a period. You should have got warning of this.
Another possibility is that your posts are replies within a topic that has been deleted by the forum owner or the original poster of that topic or the author of the post to which you replied.
Volunteer Helper - but recommending that users move off the platform!
Once the admin for GregHelp now deleted.