Based on your message, it sounds like a Search Cards filter is currently active for the board, which hides the cards if there are no results, or show only specific results. Try pressing 'x' on the keyboard to clear the search on that board, or click the 'x' next to the search results panel.
Here's some additional info on searching for cards from our help pages: https://help.trello.com/article/787-filtering-search-cards-on-a-board
If that is not the issue you are facing, do you mind sharing a screenshot of what you see?
If that would contain private information, I would advise you to create a support ticket here: http://trello.com/contact
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