If you have an external GPU connected to the PCIe slot, then it is required to connect the video cable (HDMI, DisplayPort, VGA or DVI) to the back of the external GPU.
In case you don't have an external GPU, you can plug the video cable directly to the motherboard, if this is the case, you can select "Auto" or "Int Graphics (IDG)" which should provide vide to you.
Is the case still present after following above instructions? Then please try to use different video ports and cables (HDMI, DisplayPort, VGA or DVI) this is in order to find out what could be triggering the issue (Cables or ports).
Additionally, please provide me with the model of your motherboard/Processor or brand and system model.
Hope I can hear from you.