Is it a first party memory card?
Lots of third party ones can lose data sometimes, or have difficulty being recognized by the console. Also, if the card has been heavily used or has some dusty connections, that's not going to help.
I'd take a visual look at the card's connections, and if they look clean, try it out on a friend's PS2. If that PS2 reads the card fine, it's probably your system that's got the trouble.
Which is horrible. Dead systems are a terrible feeling.
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