No memory card found but there's a memory card in it (Nikon D3100)?

I've ejected the card, put back on. But it still says "No memory card found". Help! Should I buy another memory card or is my camera damaged? Gulp. I need your help!

Added (1). Took a lot of pictures and videos this day. I don't know why this is happening, it didn't happened before. I can use the card on my computer but not on my camera!

What have you been doing with it?
is the write protect on?
did you put it in wrong way round?
have you used it in a pc? Naughty naughty.

it may need repair(software repair)

Your card might be corrupted or there's something wrong with the slot. Put a different card in and see if that works if it does then the card is broken not really much you can do about it

Could be either or both, try finding the format function on your camera and try formating your existing memory card while its in your camera. If it still says no card found then you may want to borrow a card or just get another one to try, if after getting another card thats formated by the camera in it and it still says no card found then you gonna have to have the camera checked out.

Computers are usually a bit "smarter" than cameras (cameras have limited memory to allocate to functions such as card management), so you may be able to restore the card's operation using a computer.

If the camera does not recognize the card, there's usually no point in attempting to use the format function as the camera usually has to recognize a card is inserted before the format will work.

So, try to format it in your computer and then see if it is then recognized by the camera. You must format the card in your computer using the FAT32 or exFAT file system.

Then try the card again in the camera. If it is now recognized by the camera, then reformat it in the camera so as to make sure the format is the most efficient (allocation block size, etc).

If it can't be formatted in the computer, the card may be damaged.

If it can be formatted in the computer, but if the camera still does not recognize it, it may be an incompatible memory card.

Or it might be a cheap off-brand card. While some may disagree, there really is a subtle difference in memory cards from brand to brand, and that is why camera manufacturers go to the expense of testing cards. Use the brands your camera manufacturer recommends.