Thats what I did. I moved them into the sd card and not copied them. Thanks!

Then you may have moved them. What Path do the greyed out files show (1 as example)? It may give you a clue of what happened.

On my pc

Where do they grey out? In MMW on the PC or on your phone?

First time problem. Due to losing my phone I had to put all of my music back on my s4. I chose to do it by taking out my sd card and using it straight away in my computer. When I went to transfer them, I got a dialog box saying something about modifying my files, so I agreed and not only did they not transfer now all of my music is "grayed" out and not playable. If I do select one to play, it goes all fast forward and nothing plays. I have MM version 4.1.10 and using a galaxy s4

What the hell have I done?