If the power-on indicator is not on, check the power cord for proper installation and continuity. (If the power-on indicator is not on, but the computer is functioning normally, replace the power switch/speaker assembly.) 

Check for continuity between pins 4 and 6 (switch) on P2. 
If the switch is OK, short pin 4 to pin 6 on P2 and check the power supply voltages.