If you bill 100% of the contract amount for a project and the Global Settings rule Mark projects completed when billed 100% is checked, the project status changes to Completed and the project is no longer visible in the Projects screen. Similarly, if you manually change the status of a project to something other than Active, the project will no longer be visible in the Projects screen.
By default, only 'Active' status projects are shown on the screens and in drop-down lists. The Projects screen also lists the projects that have an Active status. To view your complete, inactive or other project records, you can apply a Project Status filter in the Projects screen > list view. In your case, you can select Inactive as the project status and apply the filter. Check the Core Help for details.