It is possible that your TV was set to 4x3 resolution and got switched to 16x9. To check that setting:
Menu > Setting > Display > TV Type
If you choose to switch to the other setting, you will be shown what the TV screen will look like, and you can accept or cancel the changes.
Changing between conversion modes on Standard Definition channels and/or shows:
As you are viewing live TV, you can use the * key on the remote control (next to the zero key) to switch between Original Size, Fit to Screen, and Zoom.