Check that the power cable is plugged fully into back of computer and that a light comes on the pc when you switch it on. Make sure monitor is switched on.
Look at the back of the monitor and check that the power cable is plugged fully in. Also check that the monitor video cable (normally blue) at rear of screen is plugged fully in.
If the PC is connected to a whiteboard, then check the the video splitter (small box with 3 cables plugged into it, on floor near computer) has power and is plugged in and switched on.