Do you recall receiving a warning along with the invitation to join a group, stating that the group prevents members from having private projects? If not, perhaps your group administrator changed 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 done your group admin can change the group settings back to not allow private projects.