A common cause for this is that someone has mistakenly encrypted the files using Windows Encrypting File System encryption. The easiest way to determine this is to look at how Windows Explorer displays the names. If the file names are displayed in green text, they are encrypted with EFS.
You then need to find who did that, and ask that person to decrypt them. If this is not the problem, it is indeed an access control issue – possibly on the share itself.
You can also try to copy them from the share to the local drive and see if you can access them there. This will help in understanding where the problem is – it’s not a solution, just a way of removing the network access issues from the equation.