Not booting up after HDD install

hey guys so I made a horrific mistake somewhere by attempting to add a HD on my iMac.. I had ordered the wrong SATA power cable and so put the machine back together , and only left out the optical drive out, which has never really worked properly since I got it.. Anyway put the thing back together and now I've got nothing. No screen, no chime, nothing. Just fans spinning very loudly, and LED lights 1 and 2 lit on the motherboard. Strange thing is I had messed up the data cable for the LCD , which is extremely fragile :( but when plugging back in the smallest wire for the LCD all power shuts off and the two LED goes off as if there is NO power at all.. So I tried my second monitorn - my tv with the mini display to HDMI and nothing either..

I'm really at my wits end with this.. I consider myself to be quite handy with electronics, I've built computers and laptops from scratch and considered this tone a straight forward affair... How wrong I was.. So my iMac is sitting no screen, just the 2 LEDs lighting up and a very loud fan..

Any help or assistance on what has happened would be greatly appreciated ...

Many thanks

Tried the suggestions to no luck... Fans still spinning wildly, no display, still only two LEDs on... One thing I've just noticed, when the iMacs not powered on, there is a strange clicking noise and buzz from the power supply... Could this have been shortaged during the HDD replacement?

Also when reseating the ram I turned it on before putting them back in and I got a few beeps .. Guess that's the logic board telling me there's no ram.. And hoping by that nothing is wrong with the logic board... Argh I wished I never even bothered now!