SD Cards can go kaput. That may have been what happened.
My best suggestion is to either try your SD Card in another device or go to Bestus Buyus (or other electronics superstore) and buy a USB SD Card Reader (and you may need an adapter as well to convert what fits in your phone to the proper SD Size). You can plug that into your computer to see if your phone's SD Reader has gone kaput or if the card itself has gone kaput (If it's fine, popping a SD Card into the reader connected to your PC should pull everything up).
This goes back to something I preach but don't fully practice (although I'm getting better) -- BACK UP OFTEN. I have my photos going to Google+ automagically. My contacts go to Google Contacts... But, I have a bad habit of not backing up files off my PC... Recently my Network Hard Drive (NAS) died, and I "lost" everything on that terabyte drive. Luckily it was the motherboard in it and not the drives, so I was able to recover the data by pulling them out and putting them in my desktop.