Any boards that you create or which are shared with you will be stored within the the dashboard controls. You can select a board to view or to make changes to it by selecting 'View all dashboards' and then the board you wish to access.
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How to find a board
Within the 'Dashboard' section selected, click into the dashboard controls. You'll find this option in the top right of the currently selected board. This will bring up various options, including 'View all dashboards' and 'Search to find a Board':