overheating pc

Okay so long story short: -I got a pc in early february -Was working fine for 2 or 3 weeks or so -Then suddenly one day it just shut off and it wouldnt turn on -I called the guy who sold me it,and he "fixed it" -Once i got it back,he told me he "replaced" my 580 8gb for a 480 (witch is shady but...