You applied a filter, probably via a shortcut key by mistake. "q" is the most common one, which filters the whole board to just cards you joined (with your avatar on the front). It's useful if you're assigning work and want to see what's assigned to you.
There's a "Filter" button in the board header with a "x" that allows you to clear the filter. Alternatively just hit the "q" key again to clear the filter.
See https://support.atlassian.com/trello/docs/filtering-for-cards-on-a-board/
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