Well as they say in my line of work (You should have backed up your work) In other words, Should have bought a second memory card, just incase, and at the end of the week, copy it all over to the second card and put it away somewhere. This has been known to happen to Memory cards, not just with Gamecube, but also with PS-PS/2 and the sort.
It also has to do with your memory card's age.... after a while it will degrade..... heck, back in the Super nintendo days (Yeah, that's right, i still play it) My Super Mario World will sometimes be wiped clean, if it is removed in an odd way. I found out that even if you turn the console off, if you pull the card (Or in my case wih Super Mario World) in some odd angle or put it in not all the way and power it up, it can cause a surge (Very small one) that has the potintial of wiping your information, except all that is hardwired into the device. (ie: Electrical Surge and Memory Section of a Device = Magnet and a Video Tape)
Even though the Gamecube is shut off, it is still plugged into the wall and therefore still has power. Just be careful how you pull it out or what you put near your console. (No Magnets, or Radioactive Monkeys)