* Glitch: It's entirely possible you've stumbled upon a glitch in the game. Pokemon Yellow is known for having a few quirks, particularly in older versions.
* Misunderstanding: Are you sure you're using the card key on the intended door? Sometimes, the game has a specific object required for each door, and you might be using the card key on a door that requires something else.
* "Soft Reset" and "Hard Reset": If you think the game has truly glitched out, these can help.
* "Soft Reset" (press Select, Start, and A simultaneously) might fix minor glitches.
* "Hard Reset" (turn off your game and start it again) may be needed for more persistent problems.
If you can give me more details about where you're using the card key, I might be able to give you more specific advice!