Take no more pics until you've recovered your photos, take the card out of the camera. This is to reduce the chance of you overwriting some of your "lost" photos. There are many good freeware photo recovery software options that may help you recover your photos.
This article reviews and outlines several of the better ones. Note that you may need to use the corrupted card procedures listed in the article.
Once you've recovered your photos, you'll need to format the card in your camera, using the camera's format feature (look for it in the camera's menus). This will rejuvenate the card, but will also erase it, so only do such after recovering, or as per the corrupted card procedures of the above link. And in the future, do not use your memory card for long term storage, as they are prone to corrupt as you've discovered. Instead regularly save and backup your photos using your computer.
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