Like CopyCat said, there is no way in retrieving your posts back. Another admin on your forum must of deleted them all. Be careful on who you give admin permissions. There really shouldn't be a reason why there needs to be a second admin unless you have a very large community and need help.
The only thing you can do now is rebuild your forum and posts. If you continue to use that same forum, make sure you remove the admin permissions of the other admin.
Also, moved to correct section.