A black line forms a circumference round the display unit

Greetings everyone!! I own a computer, the maker was ASUS. When I start it up, a black line forms a circumference round the display unit, causing complications though it doesn’t make it impossible for me to view on the screen. The visual display unit does not have a button that automatically fit-tunes it. However, I can view all the contents clearly when I connect it to high definition TV. My comprehension about all these is limited so can anyone help me?
