When you open a board that has no cards to visualize (usually due to a filter that is applied), you will find the message displayed in the screenshot below.

An example where this could happen is when you set the "Me" filter to show only your cards, not the whole team, and there are no cards assigned to you.

In this case, you can remove the filters or click "Show unassigned cards and empty lists" to visualize the lists of the board.