Why won't my camera read the memory card?

For whatever it's worth, The camera model is a Nikon S3100.It came with a 4 gig card, when I bought it. Up until a day ago, it seemed to work just fine. I plugged the camera into my laptop to take a look at some photos I had taken. Later on after I had unplugged everything and had the camera on, it just stopped recognizing the card. I put a brand new card in and it still comes up with the message "This card can't be read". My computer recognizes the data on the card.

Added (1). I thought I did format it in the camera. But, when I put a brand new card in it won't even read it for me to format it. The camera doesn't seem to be able to read any card at all.

Go to store n checkout d problem from where you bought

Have you formatted it in the camera?

It's and SD card. Are you sure you aren't putting it in backwards?