Memory cards for the ps1 and ps2 are made with Flash memory. It allows them to be rewritable, but, flash memory has limited rewrite cycles. They start dying after thousands of rewrites (New cards have longer lifes, but we are talking about a flash memory of 15 years ago, more or less).

Considering that your memory card has quite a lot of years of use, the memory has worn out. Recover the data, and get a new card.