My computer won't turn on?

Hi guys I'm really hoping you can help me on this one. I was on my computer for a while and got off to go downstairs. I came back up about 20 minutes later to find out that my computer wasn't on yet the power on was still lit. So I unplugged the power cord and plugged it back in and tried to turn it on again but it seems like it tries to start up for a millisecond then turns off again. I opened my computer up to find out that one of my spare power supply cables was blocking the main fan. I'm guessing it's an overheating problem but I want your guys opinions, please help =(.