Can't turn on PC -- snowball of errors! (OS error loading/ CPU fan error/ finally it just died)

Can't give exact machine spec because I can't get it on.
AMD
350GB HDD
2 GB Ram
Windows 7 Home Premium 64 Bit


I could not figure out which category to post in so please help me!
I will give the back story and you can move the thread if you think it's in the wrong place...

Current issue is this>

I press the power button and the fan spins for maybe half a second and then stops - nothing else happens at all. No beeps, no sound, nothing. It's just dead.

Leading up to this was this sequence of events>

My pc was crashing out due to overheating.
It seemed to be that he thermal paste needed re applying.
So I bought thermal paste - followed the regular steps to apply etc, re-fitted the fan.

Powered on machine.
Saw usual start up screens with press del for setup etc.
Then> CPU fan error, press F1 to resume.
(The fan was spinning)
Pressed F1
It seemed as though star up was going to continue but instead screen went blank with just a flashing curser in top right corner.

Tried several re-starts.
Changed connection on CPU fan.
Same issue kept happening, exactly the same.

Went to bios and changed fan boot thing (can't think what it's called) to ignore.

Re-started.
Fan spinning. (no cpu fan error message)
Regulr boot process kicked in
Screen blank, then > Error loading Operating System.

Checked all internal wires etc. Unplugged unnecessary stuff like dvd sata cable etc. Replaced HDD sata cable with new Sata cable.

Re-started.
Nothing.

Powered down. Unplugged everything, replugged, checked wall socket etc.
Re-started (while looking inside case)
Fan spins for 1 second. And nothing else happens. Nothing at all.

I don't want to do anything else to the system until I have gained some useful advice from the expert folks here.

Hope you are able to assist me.

(I can't narrow down what the root cause is for this as the system worked fine apart from random crashes when it was hot. The only change I have made really is to re-do thermal paste on heat sync. But can't see why tis would affect OS loading etc. And also can't understand why machine was attempting to boot and then just gave up after I tried to address the probem of the OS not loading.)

HELP! (all my work stuff is on this pc and I can't work without it!)
 
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