Any Boards that you create or which are shared with you will be stored within the 'My Boards' area. You can select a Board to view or to make changes to it by selecting 'My Boards' and then the Board you wish to access.
Video: How to locate a Board from the main list, or using the search function
How to find a Board
With the Boards section selected, click 'My Boards'. You'll find this option in the top left of the currently selected Board:
This will bring up a list of the Boards on your Account and any folders. Another option to quickly find the Board you're looking for is to use the search box in the top right of the window: