display options

I was in system preferences and clicked on display options when my computer suddenly stopped displaying images in the regular fashion. In order for images to be displayed in the same resolution/zoom/size as before I cannot use the entire length and bredth of the screen. It is similar to a pre-HD TV size image on a modern day HD screen (ie not filling the entire screen). I've been messing around with different resolutions but I can't seem to get it back to the way it was before. Does anybody know how to make my computer go back to "normal?"