Thank you for the help!
Before I posted I had already checked if the power button cables where connected, also the motherboard has an onboard power button so I pressed that one also, no response.
Shorted the power button pins but that didnt work either.
Took out all the power supply wires and only left some fans connected to the psu, shorted the green wire with a paperclip to a black wire next to it, connected the psu and pressed the button to switch it on.. No response.
Conclusion: Dead Power Supply
Again thanks for helping out