Why Won't My Self Assembled Computer Start Up?

Question

I reassembled my computer to the best of my abilities, everything looks in order but when i go to start it, I can hear what seems to be the Power Source fan, and i can see my other fans running too; however, this only lasts for roughly half a second. My monitor also recieves a signal as well, but it stops almost immediatly and returns to the monitor to the "No Signal" screen. If anyone could help, it would be greatly appreciated. Thank you.

Answer 1

unplug everything EXCEPT motherbpard, PSU, CPU and fan, and memory.
Power on, listed to error beeps ( I think it's 3 for no VGA, assuming not onboard VGA) If VGA is onboard, connect monitor.

If this works, add in HDD
Test
Add optival drives
test
Add anything else 1 item at a time
Test

This way you'll find what is causing a problem if it recurrs.

Answer 2

this sounds like post is stopping it (power on self test). go over everything, EVERYTHING!!!!. check every screw, make sure its not touching anything and shorting it out, make sure the cables arent broken, and make sure everything is seated in its slot. this is very important, if a graphics card, or ram, or whatever isnt all the way down in its slot, it tends to cause weird things to happen.