A number of different scenarios can cause a section of the card to be non-readable, such as abrupt power-loss or attempting to remove/removing the card while files are being written to it.

As a first step the suggestion is to try on a different device to ensure that the file is available or not. 

If not, the card can be formatted, but make sure to save all other files, photos, music, video and more to a secondary location as formatting the card will erase all of the contents in it.