Make sure the monitor is on. If no power light (green or orange light) is seen on the monitor display try pressing the power button until it comes on.
If your computer monitor was on and you stepped away from the computer and upon returning it was black, it's likely that the computer is asleep.
Try moving your mouse, clicking the mouse buttons, and/or pressing any key (space bar) on the keyboard to wake it up. Make sure that the monitor is connected properly to the back of the computer.