Your post is pretty light on details, but as a guess:
Change the game's resolution to whatever your monitor is. You normally do
this from within the game's options. It is probably set on 800 x 600 and
your monitor is probably something higher like 1280 x 1024. Check in
Control Panel>Display>Settings to see what your resolution is and then make
the game settings match.
If that didn't solve your issue, please do post back after reading some of
these links to see what details to include:
