What is wrong with my PS2 memory card? How do I reformat it?

One day I decided to play Guitar Hero Aerosmith. I put the game in, started the system and I think I took my memory card out when it says 'CHECKING MEMORY CARD: DO NOT REMOVE'. Idk what I was thinking. But anyways, whenever I try to play a game, or save something, I get the message 'Error occured: The memory card has been either removed or replaced'. Something along those lines. When I go to brower, an click on the memory card, it tells me that there is no data on the card. Then when I go to save a file, I get that message but it senses the memory card. It will show up as a place to save when I put it in each slot, and it says 'STATUS VALID' on the side. So it's not corrupt. I want to try and reformat it without buying anything, but from what I hear, you can do it in system configuration by clicking one 'MEMORY CARD SETTINGS' or something like that. But I do not see anything like that in System Configuration. And since there is apparently no data on the memory card, I can't do anything by going to the memory card in browser. I heard you could reformat the card there also. The game I play is Madden 11, but the card doesn't work on any other of my games either. I really need some help here! Thanks!