SD card problem: could view photos on camera yesterday, but only those taken yesterday have disappeared?

I have a few hundred photos on an SD card. Took about 50 yesterday, and was able to look through them last night (on the camera). But today when I tried to upload them to my computer, they, and only the ones I had taken yesterday, had disappeared. What is this blasphemy? Is this a common problem with SD cards? Whyyyyy?