A few days ago I noticed that my GPU was overheating and today got around to applying some Arctic Silver 5 thermal paste to both the GPU and CPU. Afterwards the computer would not start. When I plug the power cable into the tower from the wall the machine will turn on for a few seconds and then shut down. The motherboard lights will stay on after the fans shut down and the PSU turns off. Pushing the power button on the tower does nothing and the only way to get the machine to turn on then off again is to switch the PSU switch off and let the power drain until the motherboard lights turn off-- then when I switch it on the computer will turn on for a few seconds and shut down.
First make sure all connections are properly made. If so then take it all back apart and look for signs of thermal paste in places it should not be. Clean everything with small amounts of 80%+ Isopropyl alcohol and a lint free rag. Use a pea sized amount of thermal paste in the center of you CPU and about half that on your GPU.
Also the "paperclip" test is not a 100% definite say so that a PSU is good and diablotek is a notoriously bad brand prone to failure. Just something to consider.
Turned out a pin was so bent that it just fell of, which I find surprising considering I didn't take it out.. but mistakes happen. I am going to replace it with a 6350 or 8350 and I'll be snagging a new PSU too.
So if you have similar symptoms be sure to check your pins!