Well, I sincerely hope that you have the originals that you copied to that disk. Then it's a simple matter: throw out the drive, get a new one, and copy the data to it. You should always have at least two copies of anything important.
That said. The most commonly successful solution is to download a partition recovery tool such as Recuva or EASEUS Partition Recovery and let it attempt to repair the drive's structure. This can recover from most (that's my personal opinion only) issues that do not involve a hardware issue.
EDIT: A thought. Is it showing this on the same machine you used to copy the data to it, or on a different one? If different, can you try it on the original one? If it's seen on the original one, what is the difference between the two machines? We sometimes get questions like "I put movies on my HDD and my TV won't play them" that are due to different systems using different filesystems and file types.