My Computer Is about to die.. Please help... I'm about to cry...

FIRST THINGS FIRST!
Trouble shooting specs:
1 Yes I read the owners manual and nothing helped me
2 Yes I plugged in the 4/8-pin CPU power connector located near the CPU socket
3 I installed the standoffs
4 I verified that the GPU was seated
5 I dont have any power cord to the GPU
6 Yep I tried starting with one ram stick installed
7 The RAM is fully seated
8 Yes, I verified that i am using the correct RAM slots
9 Yes, I removed the plastic guard over the CPU
10 I installed the CPU correctly
11 No, I only got thermal paste on the CPU surface
12 The CPU fan is plugged in
13 N/A I'm using a market CPU cooler
14 No there aren't any loose screw lying about the case, they are all screwed in.
15 I Discharged myself before I started by touching a grounded object.
16 Yes, I installed the system speaker/beeper
17 Sadly I didnt read the manual on where to put the plugs, It got chewed up by my dog, but I had my friend show me
where to put them, plus they are labled on the mobo.
18 Yes, of course I had the power turned on.
19 I've been using the same CPU the whole time, i.e its supported.
20 Yes, the first thing I tried was resetting th CMOS by taking out the battery for 15 minutes
21 Yes, I've tried the intergrated video port, and i've even tried using a HDMI cable, but I get no signal
Overall:  
ACTUAL PROBLEM!
I have a computer, its about 3 years old.(I constantly upgrade it.)
Specs:
320 Gb hdd
4gb ram
win 7 ultimate x32
Sapphire Radeon 5450 (20watts)
Intel Core2Duo E6750@2.66
Gigabyte GA G31m S2l
350w PSU
Coolermaster Elite 331
My problem is as follows,
A few days ago I found that my CPU was over heating. So I got out one of my spare CPU Coolers (Coolermaster 212)
and installed it.(Note I took the mother board out)
When I had finished I tried to start the computer by pressing the on button and nothing happened, I realised i plugged th PWR_LED cables into the wrong places, so (I think) I corrected them. Now It turns on but I cant switch it off i have to use the switch on the psu. But my problem is that i dont get a signal to the monitor, the fans turn, and my mouse (cable) does not turn on unless I use a USB PS/2 adapter. My kewyboard doesnt turn on either. What could br the problem? |Have I fried my PSU or my mobo? I just don't know what to do...