Hello and welcome to Android Forums. Hopefully I can help you with your problem.
Try putting the sd card in a computer and seeing if you still have the files. There is a chance that the phone isn't reading the sd card and your computer will read the sd card.
There is also a chance that your sd card went bad too. If that is the case then your data is pretty much history.
If you phone just deleted all of your sd card for no reason, you need to send your phone into Samsung for warranty repair. Seeing as it's an S4, you should still be in the 1 year warranty window.
If you have any other questions regarding your device, I invite you to join Android Forums. We have many sections for specific devices which contain a wealth of information.