It could be that something has gone wrong with your memory card. The best thing to do would be to run a recovery software on the card to see what it can find. Here's a couple of guides, one from
Have Camera Will Travel and another from
CNET. Have Camera Will Travel's guide includes free and paid options, CNET's look to be free and has a more detailed guide. If you're not comfortable doing it yourself, see if you have a photolab where you live that offers a photo recovery service. The photolab at the store I work at offers one, though we're part of a small western Canadian retail chain so no guarantees for you, but it's worth a shot to check around if you aren't comfortable doing it yourself.