|Tell us the make and model of the monitor .|
What program are you using to play the movie?
Programs that play movies often default to not showing a movie in full screen mode when you first start the program - you have to click on something, or RIGHT click on where the video is displayed and choose full screen, or change a default setting so a movie always starts up in full screen mode.
E.g. in Windows Media Player if you move your cursor so it's over symbols below the right bottom corner of the box where the video is displayed, it will show you which one is for full screen.
Depending on the resolution of the movie in comparison to the resolution of your monitor display (the ratio of width to height), you may have portions of the full screen display that have no video (blackness) , top and bottom, or left and right. If you have no video (blackness) all the way around in full screen mode, you need to change the resolution of your computer monitor in Windows.