Why won't my phone register memory card

I have had my Samsung galaxy s3 for a little over 2 months. I moved my memory card from my old phone with all my contacts and music, (and have been playing my music). Today for no apparent reason my phone started playing up, telling me it cannot support the audio files and flicking through my music files rapidly. I removed the card, switched off the phone and restarted it, now it won't register the memory card. All my contacts and music have disappeared. What do I do?