Computer poppped and turned off

so after hearing a loud pop, my comp and everything just shut right down...took my power supply apart and found a little tiny screw arcing off another screw down on the board in the psu..i have a corsair hx850...there was also a little washer in there..im guessing from my son dropping stuff down in it, or me dropping it in there when i was putting the PC together, all capacitors on the mobo look fine, all the ones in the psu look fine, but now when i go to plug the psu in, no mobo light comes on, along with the gfx card...the arc left a melting little black spot on the board in the psu and the screw was black lol...so dead psu??