You probably just have to fit your source to the screen. For example if its a monitor or window capture, right-click it in the list of sources in OBS, then in the Position/Size menu, select "Fit to Screen".
On gamecapture it can sometimes be needed to activate "Stretch image to screen", thats if you right-click your game capture and go into its properties. If that does not help, show us a log file:
Problem? Make sure to post a log and/or crash dump -- HOWTO
That will tell us a bit more about your system and configuration of OBS, maybe that gives me a hint. Oh and if you are new to OBS, you might also want to checkout our help file, to learn about moving and resizing sources while "edit mode" is active, choosing a correct capture method, etc.:
http://jp9000.github.io/OBS/