First, make sure your monitor is powered up and correct input selected using the monitor's built-in menu system (often called the OSD). If that doesn't help, turn off the PC and unplug it before opening up the side panel and checking the power connector(s) fit, the graphics card seating is firm, and you have plugged the monitor into the new graphics card and not, for example, a motherboard provided video output. If everything checks out, please consider your power supply might be insufficient for the upgrade you have chosen.