It's possible you might've changed the program used to open image files by accident. So .png images do not have a image viewer program to open with.
Go to your start menu (or press CTRL and ESC at the same time) and type in Default Programs then it should be first on the list. Click the Default Programs icon. It should have opened up a part in the Control Panel, choose the option Associate a file type or protocol with a specific program.
Scroll down until you find .png and then click the Change Program... button. Choose what you would like the image to open with. For example I would choose "Windows Photo Viewer" but it depends what you usually open .png files with.
After that click Ok and try opening the image again. Hopefully it should work ^^
