Look at the back of the power supply and try to find the power switch (a rocker switch that has a vertical bar for on and a circle for off). If the switch is off, turn it on and try to boot the machine. If the switch is on, turn it off for about a minute, turn it back on, and then try to boot the machine.

If this doesn't fix the problem, it's possible that your power supply is shot. There are a number of other possibilities, but without being there, it's very hard to diagnose the problem.