as for the initial page of the application: decide which page (standard diagram, freeform diagram, etc.) you want to be your template document > File menu > Set Global Default Diagram as (name of the diagram); next time you open the application or a new default diagram, you will see that particular page.

as for the page with your project open: File menu > Set Project Default Diagram as (name of the diagram); next time you open this project you will see the diagram previously created in the main panel. Otherwise, if you open a project which was not set as default, you will only see the list of entities making up the diagram in the project tree on the left of the page.

To continue working on that diagram you need to recalculate it first (see paragraph 4).