I would first check the outlet and line cord to make sure you have power getting to the system. Then check the system socket for damage.
With the cord disconnected from the AC outlet I would trace the cord contacts to the socket lines from within the case where it connects to the power supply using a continuity tester (not power).
Then carefully test the power output at the logic board header you should measure the DC voltages per the schematics and board view. Are the present?