Normaly, there is a green/amber led next to the power button. Does it light up?
If it stay dark, the monitor is probably not connected to any power outlet. Connect it to the wall. Maybe the power cable is missing or damaged. Replace the power cable or get one.
If it goes amber, it usualy mean that there is no signal comming in. Check that it's correctly connected to a computer or other video source. Check that the cable is intact. Replace the cable if it's damaged.