sd card photos were disappeared while downloading to hard disk

[Ash]

Can you tell us more?
Were the photos in the process of downloading then had a corrupt file or are none of the photos visible on your computer?
Can you see any of the photos using your camera? If so, I wonder whether the card is OK but your computer has an issue reading the card?
The SD card is commonly the culprit, but it is good to rule out anything else. If the SD card is corrupted, there is little you can do to fix the problem other than to replace the SD card with a better quality one, such as SanDisk.