Bad memory card - damage to the connectors or simply a faulty card.
If you're done viewing images on the computer, it is not well enough to delete images from the computer settings. It's highly recommended to also format the card in the camera for a clean wipe.
Turning your camera off and on too quickly before the image can transfer from the internal camera buffer to the card. This can cause a corruption. After taking an image, give the camera a few seconds before turing it off.
Be sure camera if OFF before removing the memory card.
Took a few (or a lot) of images and now my camera tells me something is wrong and asks me to format card. Don't format! Change to another card and continue the fun. Chances are the images may be recovered prior to the corruption event.
Oppps, I formatted my card and lost the images. All is not lost! The images stand good chance of being recovered at Crivello Camera Center in Brookfield, WI.