The SD card likely got corrupted - this happens if you don't disconnect it properly from your computer when you put music and stuff on it. It can also happen if the device has had a long life already - flash memory can only take so much abuse.
Take the SD memory card out of your phone and find an adapter to plug it into your computer. If it works on your computer, try and find a data recovery software. Don't write anything to the flash memory from your PC at this point because you won't be able to recover anything.
Flash memory is kind of like carbon paper. There's two layers and when you write on the top layer, the image is imprinted on the bottom layer. If you were to delete everything on the top layer - it would still be visible on the bottom layer. However if you were to then write on the top layer again it would start copying the image to the bottom layer and thus write over the other information.
So like I said - try it in your computer. If it works, try and recover the data using a data recovery software.
If it doesn't work in your computer you'll need a new SD card, and unfortunately all your data is lost. In this case it means the flash memory is corrupted and everything is gone.