Computer won't start

Hello,
Today I just wanted to boot my PC as normal, but nothing happened when I pressed the power button. The only sign that the motherboard got energy was that the lan lights were on. Otherwise there were no fans or harddrives running. I tried to change the power cable but that didn't help also I cleared the bios by cmos but that didn't work either. So at the moment I am a bit clueless what to do now, because I have no idea if it is my PSU or my motherboard which causes these troubles.

Any advice on this is greatly appreciated.

Thanks in advance