Hello Natasha.
It's like foning your favorite car mechanic and telling him/her that your car won't start and asking what you can do... His first question will be "Whats the make and model?"
So the first question about your monitor is "What's the make and model?"
This fault is 'generally' caused by damage to the connection cable, or incorrect plugging but may be caused by heat stress or a video card problem. We could accept some details of your pc as well, the more info you supply the better chance of us being able to come up with a reasonably sane diagnosis..
Thanks.