In Cubase you can have several projects opened at the same time. However, only one project can be active. The active project is indicated by the lit Activate Project button in the upper left corner of the Project window. To work on another project, activate the other project.
Procedure
To activate a project, click Activate Project
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Note
If you close the active project, you must activate another open project manually, as Cubase cannot automatically activate one of the other open projects.