Most monitors will automatically select the source that is sending a video signal, but sometimes the wrong one will be selected. Other times, you will need to manually select it. You can change the source with the buttons on the monitor or in the on-screen menu.
Some Samsung monitors have a power switch on the back in addition to the power button on the front. If there is a power switch on the back of your monitor, make sure that it is turned on, and then press the power button on the front of the monitor.