With only "new computer screen" and no maker or model or what sends it video signal, no accurate guessing can be done.
Hope someday you might realize how important ALL DETAILS are when asking for long distance help here at Answers.
This means you do not have an input device such as a PC connected to it
You may have a device such as a PC connected to it but you have the wrong "source" selected. (such as HDMI 1, HDMI2, DVI) or whatever port you have your device connected to.
Choose the correct source by navigating through the input options on your monitor and select the input your device is connected to.
or as William said if you are using a dedicated graphics card make sure you monitor is connected to it and not your motherboards on-board graphics.
Your motherboard ports are aligned vertically and are on the top of the PC
If you have a dedicated graphics card It will be lower down and will have around 3 or 4 connections on it they will be aligned horizontally generally.
Make sure it's plugged into the GPU (if you have one) and not the motherboard. If your computer has a GPU, there will be 2 sets of video outputs: one surrounded by a bunch of other ports like USB, ethernet, and audio and one that isn't near any other type of port. Plug the monitor into one of the ports in the second group.
is it connected to computer