When you WERE able to get into the memory card still, did you ever notice a block of corrupted data and did you delete it? Also, have you ever used cheats and then saved while using the memory card? There are some games which may actually corrupt a memory card if you use cheats extensively, then save that game.
It happened to someone I know. But, the fact that you can still save to the card does sound fishy, since if it were completely screwed, that would be it.
Try it on someone elses PS2, if it works on theirs, then it may be your memory card ports are going on your PS2. I've seen it happen before. There are a ton of places online that sell replacement card/controller ports online too, just google it and you should get quite a few to come up.
While I've never used this site for any parts, they have free shipping on all domestic orders and most international ones and they even have a part finder which shows you pics of what your PS2 looks like, so you can get the right parts to fit you system.
But, an FYI, MOST larger PS2 models are the same from 30001 to 50001 and the parts are mostly interchangeable.
Site to check for parts, should you need controller/card ports:
http://www.richspsxparts.com/