Freddie Sotelo :
What do you mean by start up? Does your system POST(Single beep from the motherboard)? Are there multiple beeps? Can you see the BIOS? Check your connections. Make sure everything is plugged in correctly. If you can, try to test some of your parts on a known working computer. What's your motherboard model? I know this may be stupid, but make sure the power switch on power supply is on. Let me know what happens.
My motherboard is an Intel H61BF, and I have done the paperclip test with my power supply at this point and it is functional. I have no idea what is wrong now, as when I hook everything up and switch the power supply on and press the power button on my case (it is also hooked up correctly), nothing happens, Nothing at all. No lights or movement whatsoever.
EDIT: I took everything apart and rebuilt outside of the case. This time, I made sure the 24-pin power cord was plugged into the motherboard all the way, and it started to run. The CPU fan, case fan, and power supply all started working and the CPU started to heat up. But, after about a minute, it would all stop. Nothing appeared on the monitor during this time either. Also, I did not have the GPU hooked up this time. I thought maybe it was taking up too much power.