take the power supply out and on the 24 pin atx cable with a paper clip jumper the green wire to a black wire. then turn the power switch on on the back of the power supply. if the switch turns off on it own again there may be a dead short in the power supply. on newer units should auto change there voltage for us or over seas voltages. some of the cheaper units and older units have a red 110/220v switch on them check if your in the us the switch is set to 110v not 220v.