I have a Canon G12 which takes great pictures. I went to take a picture the other day and the LCD screen did not completely fill out with an image. If I look through the view finder I see more than on the LCD screen. The LCD screen is black on both sides by 12mm whether I am trying to take a picture or view a picture I have already taken. When in picture taking mode the black sides do have some some camera settings info on them. But that info was also there when screen was full across with a picture. How do I get my full screen back again?
Most probably you have changed the image ratio from 4:3 to 16:9 (or vice versa) and now you have black stripes because screen ratio doesn't match your image ratio. Set the image ratio back to 16:9 (or 4:3 depending on screen ratio) and the problem should be solved.
I once had a Canon G12, and if I recall right, the LCD display uses the full display area when the aspect ratio is set to 4:3. It then means the aspect ratio has been set to 1:1 or something else than 4:3.
So, just as Alex S said, change the aspect ratio. On English manual it is explained on chapter 'Changing The Aspect Ratio' (probably page 78).
EDIT: Make a web search on "Canon G 12 manual" to find a PDF manual for your camera, if you no longer have one. I'll always download the electronic versions of manuals for my cameras on some cloud service, to make them accessible for example on my smart phone.
Viewfinder accuracy has always beset the photographer. An optical viewfinder is independent of the camera’s taking lens. Because it is off-axis it displays parallax error (image position offset). Additionally because it is independent, the magnification will be different as compared to what the sensor sees. The SLR (single lens reflex) design is preferred when it comes to accuracy, Looking up the specification and user posts for the G12, the optical viewfinder only show 78% of the actual view while the LCD view is approximately 100% correct.