2. Step 1 - Don't Overlook the Obvious!
Some of the most common problems are caused by cables which are loose, faulty, or which were strained/unplugged by the office cleaners, or borrowed by the person in the next room while you were on holiday. This can result in equipment that appears dead, screens that are blank or have strange colours, or workstations that can't contact the network. For example:
The various controls on the computer, display, sound system etc. may have been changed by another user. For example your screen unit may power up and the cables are all OK but there is still nothing on the screen. The cause could be that the monitor's brightness and/or contrast control have been turned right down and just need re-adjusting.
Another (real-life!) example: many computers provide the mains power for their accompanying display - this means that the display's own power switch normally can be left in the "on" position and the computer's own power switch used to turn both computer and display on or off. Someone else using such a computer might "helpfully" turn off the display's power switch so that when the system is next turned on, the display appears dead until someone notices that the display needs switching on.