What's wrong with my nikon? How much does a 8g memory card cost for it?

I have a nikon D3100 and like out of the blue my mom grabed my cam to take a photo and a little green light came up i took it too look why and i turned it off turned it back on then it deleted about 100 of my photos. Then it said my memory card might be damaged and i needed a new one. So i'm guessing thats what's wrong with it? I'm not sure though because it doesn't turn off and freezes now since it said that about the memory card. But i wanted to know how much it cost for a 8g memory card for my nikon?

Around $25.

Cost $20-30

Take the card out - will the camera turn on and off? If no then the camera is bust not the card.