When you receive an invitation to join a group, you automatically get an alert which warns you that some groups prevent members from having projects. If you do recall seeing this warning than that is the reason to why your private projects are no longer visable. If you didn't happen to receive the invitation this could be due to your group administrator changing the settings to prevent group members from having private projects after you join the group. If that is the case, your private project folder gets removed from your projects page. To again access your private projects, please contact your group admin and ask to change the settings temporary to allow private projects. Then, you can access your private content and move it to a group project. After this is completed your group admin can change the group settings back to not allow private projects.