You can't fix the image you showed us, but we could maybe explain why it looks that way if we knew
Image mode and bit depth? Pixel dimension of the image?
Did you produce the image? It looks like a photograph, so if that is the case and you took the photograph, was it a RAW file or JPG? How did you process it?
One thing that occurrs to me if it was a RAW file is that you can set the size it opens at in the drop down at the bottom of the ACR window. If that is not set to the default size (matching the camera sensor resolution, then that could be it.
Bottom line is what is the image size in pixels? But pasting a screen-shot of your full workspace showing the layers panel might help.