If you can tell that "power is good to the mobo", please let us in on the secret lol.
PSUs have many failure modes . . . fans will run or not, lights will light or not, voltage will read 12V where it should or not . . . but the PC doesn't run and it still turns out to be the psu's fault.
First thing to try is swap in another psu, or use your psu on another PC. Since you have given us few specifics, odds are good that's your problem.