I assume the images are OK on the camera in which case the issue lies in data transfer from the card. If not OK on the camera you have a defective or unformatted card or a dirty connector on camera (see below).
If OK on the camera this could be a physical problem with the card (cracked track somewhere, unlikely as this would most probably just fail). Most likely it is a dirty, damaged or loose connector or connection and we have to eliminate error possibilities. The candidates are the camera socket, the card, the reader and the connection from reader to computer; be aware the problem may have multiple causes.
So ensure the card connectors are clean and not damaged. Inspect the reader (and camera) socket and also ensure the contacts are clean and not damaged. You are looking for bent connectors, cracks or dirt. Also do this to all USB leads, swapping leads if you can, also try a different USB socket on the computer. Finally, try a different card reader (or connect camera directly to computer) in case the connection between reader and computer is bad.