PC won't post

So I had spare parts laying around to build a 2nd PC out of it. It's basically a computer I used ~7 years ago myself. It was a prebuilt Acer Aspire m5201 (4x 1gb DDR2, Phenom x4 9100e, 640gb wd green) with additionally installed Chieftec 400w psu and 8800gts.

I finally got it together and booted it up, front panel turns on and power button work normally, HDD sounds like it should, the chassis and gpu fans turned on(at 100%). The issue is CPU fan won't turn on (I tested it on sys-fan slot and works fine) and the computer won't post at all.

Something to note is I've basically kept the CPU on the mobo for ages. I did not have a case for it so I just figured it'd stay there. Before installing the fan I cleaned it from old thermal paste and applied a new one. When I noticed the PC wasn't posting I cleaned it and reapplied it again and I also cleared the cmos. I also removed the gpu and used the onboard chip but again nothing changed. Edit: Took off RAMS, had no beeps or nothing. Tried a few stick configurations again no change. I also tried using LED fan on the cpu fan slot and the leds lighted up but the fan wouldnt spin.

What could the problem be here?

I don't understand what PC won't post mean. English is not my mother language..... but if you change the title of your post to a better description you're more likely to get help.

ok, then I understand :)

I still think you should edit your initial post and modify the title so it's easier for everyone to understand.

It is best to recheck all connections. Even better to take everything out, check them for obvious signs of corrosion or damage, and put it all back together again.

Have you plugged in the 4-pin ATX 12V Power connector from the power supply to the motherboard?

If The cpu fan wont spin at all; Try and see if you have a spare fan to test the fan slot on the motherboard with. Because it sounds like it could be either the connection on the motherboard or the fan itself. If you use a spare working fan to test the slot and the spare fan works; then its most likely the fan thats bad even if it worked before. Also; if you can; test the voltage going to the cpu fan slot on the motherboard to make sure its getting juice going to it. For that you need a volt meter.

Did you pull the processor and check the pins?

How about that 4-pin connector? Check it and look for signs of melting or scorching.