[AC Question]

I took pictures back in April and suddenly in June for seemingly no reason some pictures have a black block running through them, ruining the image. There doesn't seem to be any rhyme or reason to it. The thumbnail view in the gallery looks fine and then once it is expanded, three fourths of the shot is obscured by a grey and then black rectangular. Looking at the details of the picture the picture is still large, even though the image has been reduced by a quarter. How do I fix this? And why did it happen. I now don't trust my phone camera and have uploaded every image to an online site, just in case it is something that is creeping through my pix.