Verify that the monitor is properly connected in the back of the monitor as well as to the back of the computer. If the monitor appears to be connected properly, verify that a cable is not loose by disconnecting all cables that can be disconnected in the back of the monitor (generally the data cable cannot be disconnected). Next, disconnect the data cable connected to the back of the computer and then reconnect the cable. When connecting the cable in the back of the computer ensure the cable connection is tight. Most Computers will also have screw ends that can be screwed in to hold the connector in place.