Hi Mmuraszko - welcome to the community!
It sounds like you might have a filter enabled on the board. If there is, you can click 'x' on your keyboard to remove the board filter. You'll also see a filter button at the top of the page.
Here's a guide with more information about the board filter: https://support.atlassian.com/trello/docs/filtering-for-cards-on-a-board/
I hope that helps!
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