I took my computer apart and now it's not working...

[YellowPlum928]

I recently took apart my computer to add a hard drive cage (because I have a mini itx case and it was necessary). I then put it back together and it would not work. I checked all the connections. I took the cpu out and put it back in and applied new thermal paste. The power supply is working because my front io lights up, and my graphics card is receiving power. I also noticed that my peripherals were not lighting up so the mother board usb isn't working. This leads me to believe that my mother board died. It was working yesterday but something happened during this process and now it is not working, so I know all the components work (unless I accidentally damaged something). Help, I would love to hear any suggestions! Thanks.
Edit: This is my fourth computer I've built.

 

[Karsten75]

http://www.tomshardware.com/forum/274745-31-step-step-guide-building

http://www.tomshardware.com/forum/261145-31-perform-steps-posting-post-boot-video-problems

http://www.tomshardware.com/forum/288241-31-buyer-guides-troubleshooting-tips

 

[Tchota]

make sure you have all the popwer cable connect, specially the CPU and the GPU power cable.

make sure that you are connecting the monitor to the GPU and not to the motherboard

 

[YellowPlum928]

I do have my cabel connected to the GPU, and I've check all the connections atleast twice

 

[Tchota]

if you are sure that the CPU power cable is connected, try to unplug the new HDD, from motherboard and power supply.

Make sure the CPU fan is also connected