Try connecting the phone to a computer and see if you can open the files through the computer. You can also then check the file extension of the ones that open. If they were not made on the phone (not photos you took) then they could be of a different file type that your phone won't open. You would then need to convert them over to a type that the phone can actually view.
Now if you can't even access the files from the computer, there may be a problem with where they are on the device or if on a SD card.