i suspect this likely happens during file transfer. do the images look like this on camera LCD or only on PC?
check your card reader connections and consider trying different SD cards to see if it a phenomenon of the card or the connection or your computer.
If it happens in camera, it is likely your image processor has become damaged, not the sensor. essentially the 1's and 0's that make up digital images are being repeated for the rest of the image because it has somehow become corrupted and the image cannot finish.
you could also be having hard drive issues on your pc if none of the above are your issues.