When you open a closed project, none of the Timebox items that were created while the project was closed will automatically show up in that project or it's children. Timebox items are milestones, releases, and iterations. What you can do is manually create the Timebox item at the highest level of the re-opened project with the same name, start date, and end date. Make sure the "Create matching Timebox in all child projects" is selected. Now the re-opened project and all of it's children will be associated to that Timebox type.