Computer turning on and off after rearranging pcu cables?

last night after hooking up my new mouse keyboard and monitor ( which work fine ) i decided to tidy up my cables as i have a big window in my case, after a hour of deciding on the best layout i screwed everything back together only to find that now my computer wont start properly! it turns on ( fans, hard drives, ( no beep) but it seems that as soon as the motherboard tries to power on ( i say this because the cpu fan just starts to spin) it shuts down. and then after 2 seconds it powers back on again with the same results. i have checked for stray metal arcing out on the case and found none. i have removed and re-installed all cables numerous times, i have tried removing RAM disconnecting hdd's booting with only motherboard and graphics card. read all the other forums and tried everything. please please please any advice would be fantastic as im about to hang myself with my ac cord