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.
You can find inactive projects using the Project Status filter on the Projects screen. On the Filters panel, select Project Status from the drop-down and set it to Inactive. When you click Apply Filter, you can see inactive projects in the grid list. You can also find inactive projects within the master project of a phased project. Check the Core Help for more on applying filters.