Firstly welcome to Seven Forums.
Even though you did the paperclip trick,and the psu made your fans run,doesn't mean the psu is "working" For example,I have a separate psu that burnt something on it years ago, it works fine for powering up a pump,or running 2 fans, but anything more then that will show it has power,but wont turn on. Chances are good that you fried something in the psu,and it still gives enough power to show you its there,but wont boot anything up.
A second scenario would be that the motherboards pci slot has been damages,however,ii you remove the gpu it should start up fin,if it doesn't,then we are back at the faulty psu.
If you can,put the gpu in another rig with another psu,motherboard and so on,if it starts up then its not the gpu, do this with the motherboard too,but don't use any gpu,just use another psu,if it starts up,you need another psu, if it doesn't then your gonna have to look allot closer on the motherboard.
I would suggest swapping the psu first,then test the rest. If it's the psu then at least it's an easy fix. Please report back after the test so we can take it from there.
But the bad news is it sounds allot like a motherboard problem,more specific like the voltage regulator got fried, it happened to me once before,same symptoms of it being switched on for a second then never again, I hope it's in your case just the psu, but chances are better that it's the regulator on the motherboard