Q: My computer won't start!
A: Are all the cables securely connected to the back of the computer and to the socket(s) in the wall?
Did you press the computer "On" button?
Is the main power swith on the PSU set to "On" or "1"?
Is the monitor on?
Is the video cable connected to the back of the computer (the cable with three rows of pins)?
Some computers may go into a "sleep mode" when not used for a while. Pressing the "Esc" button usually wakes the computer. It might take up to a minute though.
If the above doesn't resolve your problem, call your Sysadmin!