I inserted the SD card into my computer to look at some photos. I rotated some of them to look them from the correct angle, but unfortunately Windows Photo Viewer auto-saved the rotation. Now if I insert again the SD card into my Coolpix P50, when I try to visualize the photos that have been accidentally rotated I receive the error: "File contains no image data". I tried to rotate them again to the original orientation, but the error persists. Anyone knows how can I solve this problem and make my camera show again those photos?
Cameras usually do not like other software modifying their files. Camera firmwares usually able to create files and handle their own file formats, but they are not prepared to be fully standards compliant. Most cameras are extremely sensitive to even filesystem changes on the memory card, done by a computer.
Another error can be if you did not close your memory card properly, and since your computer modified the files, there were changes pending to be written. If those were not flushed, you might have ended up with damaged images.
The correct flow always is: shoot, download to computer, format memory card in camera.
Now, if your files are damaged on the memory card, and you do not have a local copy on your computer, you can try e.g. IfranView, which could open many damaged JPEGs. There are also tools that can recover files from your memory card, if they are valuable - but this is usually not a very fruitful approach.
There is probably something you can do but I'll venture to say it is not worth your time.
What happens is that cameras are programmed to show photos they take and assume that all the metadata is exactly the way they left it there. If you make any change, the metadata may change which is what happened in your case.
If you truly want to fix is you can use a command line tool like jhead to extract the metadata from a correct image and apply it to a broken image. This will of course cause other parts of the metadata to be incorrect. So you won't get your image exactly as it was unless you figure out exactly what metadata changed and use an EXIF editor to fix it.
Fuji camera can see images with changed metadata and display them with a little icon of a wrapped-gift. They cannot zoom in our out of them in most cases and usually wont display image info either.