System did not start up help


Jun 21, 2012
I have had my computer for around 6months it is the first computer i have ever built
the specs are

CPU: i5-3750k
GPU: Evga gtx670 ftw
MOBO:ASRock Z77 Extreme4

The problem was that when I pressed the on button to start up my pc nothing happened except the little light where the Ethernet cable plugs into turned on like it normally does so I held down the on button untill the light turned off ,waited a bit then flipped the power switch to off, opened my case, and wiggled the cable that connects my mobo to the psu and made sure it was in right then i flipped the power switch on and my computer turned on just fine like it normally does no problems then i started thinking if it was just that cable that connects to my mobo then why didn't the fans turn on and why did the Ethernet cable light turn on

so i wanted to know why this happened and if i was at risk for any thing

also am I suppose to flip the switch on my psu to off every time I turn my computer off?