Reinstall your video drivers. This may be your solution but you will want to check all of your drivers to make sure they are updated. If you have recently installed anything then delete that and see if it works. Disconnect your monitor and see if the connection is dusty and ensure that it is securely connected with the cable. For more troubleshooting tips check [https://guideusermanual.com/product-name-latitude-e6420-manual&po=344640&lang=English|Dell Latitude E6420 Manual.]