Thanks a lot! I'll try this tomorrow. One question: is it possible I've lost all my data on my HDDs?
I highly doubt it, however as my boss has tuaght me never say never cause you never know. But yes try a barebone boot up, maybe even without the GPU. If it boots up then its not your PSU. If you discover this is the case then slowly one at a time plug in each card/device and if the PC boots up contiue to the next one.
Where is your internet cable hooked up too? If its your MOBO I would suggest bringing it to a PC shop, kindly ask them if they would be willing to test it out for you if you don't got a spare MOBO laying around.
I'll ask my boss for you tomorrow