SD card not reading in camera?

My SD card was just working fine till yesterday. But, after i copied my pictures to computer and again re-inserted back to camera, it's not working. My camera says that SD card is not inserted when I have inserted perfectly. The thing is when I checked my SD card in my laptop, it's showing in my laptop. All photos are there too.So, I took another card and inserted in my camera and that works well but again when I insert my previous SD CARD the green light doesn't show and it just doesn't work on my camera.

What should I do? My camera is Nikon d3100

Remove the card and clean the dust and replace the card then definitely it will work

Format the troublesome card. If errors come up, throw it away. Nothing else you can do. Replace with a new one and format before using.

http://keerok-photography.blogspot.com/2011/05/how-to-keep-your-memory-card-healthy.html