ive had something a bit like this, i had rented out silent scope, played and saved fine, the next morning i loaded it up as normal, i completed it with a huge score, when i went to save it, it said i had no memorycard inserted, i pushed it back in, still no joy, cleaned it by blowing into it, still didnt work, i reseted the computer with nothing in but it said i had no memory card, i took it back to the shop and got a whole new one. this one is fine. Maybe you turned the power off or took it out as it was ssaving as this can ruin your save data