Once you have created one or more Workspaces you can always open them again, if you have no other Workspace open. Go to File|Open Workspace and select the FOLDER where the Workspace is located.
Aquabase will throw an error if the selected folder is not a valid Workspace. Once a Workspace has been opened the File menu item will change to Database.
You may also use the File|Recent Workspaces menu item where the most recent 10 Workspaces are kept for convenience.
If you open a Workspace, which points to a SQLIte/Spatialite database, which does not exist (or has been renamed) as in the “Database” location setting of the workspace.ini you will be asked to locate the folder where the database resides.