Did you try to take a screen shot of a YouTube video? I did this and it didn't turn out right, and I had the same issue ever since. I didn't realize at the time, but I hadn't played a video using the YouTube app since, either. I opened the app, watched a random video, closed the app, and then I was able to take screen shots again.
I'm not sure if this is the case with your phone, but I think it was with mine. All the screen shots I had taken all showed as having a file size of 0.
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