Screen and stuff are not showing anything

So i built my pc a while ago and i noticed my power supply should be flipped over (because it was pushing hot air at my video card) so i flipped it and endded up rerouting all my cables and moved my hard drive up for airflow. after that i plugged my hdmi cable back in and all that stuff powered it up and all the fans were spinning but nothing was on screen and my mouse and keyboards leds were not on. what might be wrong.

Computer specs:
mobo: asrock 990fx extreme3
cpu: Amd fx 6300
ram: corsair vengeance lp 2x4
graphics card: Sapphire Amd hd 7870 (oc edition)
power supply: corsair Gs 600
hard drive: 1Tb Wd caviar blue
the disc drive is a cheap Lg one