Why won't my computer turn on

I've had this computer since it came out. One day, it was sitting on the table. It randomly shut off. I took it to the repair store and they said, "it's beyond repairable." I don't believe them. I've tried using the heat gun on the CPU (like PS3 ylod) still doesn't work. Please help.

(P.s I would also like a tear down guide please.)