I think your machine's JPEG viewer has been broken. Try viewing a few images on say Imagur.com.
Hi, I'm hoping to get some help in fixing .jpg files on my computer, which are currently messed up. I *think* I did it myself, by accident, but I'm not entirely sure. For reference, I'm using Windows 7 as my operating system.
I was getting ready to upload a .jpg file to a website, when after clicking it and getting the preview version of the image showing, I realized that I needed to make one final edit before uploading it. So, I tabbed into my paint program, made the edit to the file and then saved. Then I tabbed back over to my browser and clicked to confirm the transfer, but it wouldn't work. I went back to my paint program and I couldn't save the image anymore, so I copy pasted the graphic into a new file and was able to save that. I noticed at that time, that the original file was no longer in the folder.
I uploaded the new file, thinking "huh, that was odd", but now I have found out that all of my .jpg files are now unrecognized. Trying to open them in photo viewer gives me the following error message: "Windows Photo Viewer can't open this picture because either Photo Viewer doesn't support this file format, or you don't have the latest updates to Photo Viewer."
I tried opening them in browsers, in various paint programs, all to no avail. They simply aren't recognized. I tried it with the new file I had just made and it was perfectly fine. I have made new .jpg files and they are also just fine. I started looking at what is different between my original files and the new ones, and I notice that while the sizes of the files are there, in the details section of the properties for the images, they no longer show image dimensions, bit depth or anything else like that.
I did a little bit of googling, and based on the reading I did I think that maybe somehow the file headers for .jpg files got messed up or something. I tried copying the files to my laptop, but they are still the same there. Every .jpg prior to that file upload is "broken" while every .jpg made after is fine.
Does anyone have any ideas on how I might resolve this issue? It also affected my "backup" USB drive which was plugged into the computer at the time, so I've got like 7+ years of family images and such that are currently unviewable, and I'd really like to hope that they aren't gone forever.

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