Try to clean the Circuit contacts on the memory card, and try it again. A pencil eraser does an outstanding job on clenaing the Circuit board connectors.
If that don't work then...
Run the ps2 without a disk in it and go to
browser and then press x in a little while you may see all your games that you saved.if you want to delete a save game you use the directional keys to go to the game and then you press x.
If they do show up then delete your oldest saves until it's ok.
Or logically your last save which may have caused the problem.