Dell Vostro 1500 won't power on

Hello,
After wasting a hole day in research manuals and forums I wasn't able to find a solution for my problem.
So here is the problem - after a normal day of use I left the laptop alone turn on (as usual) but this time when I came back it was turned off and I never managed to turn it on again.
When I press the power button on it the only sing of life (if you can still say there is something living) is the blue power indicator on the side. (the power indicator stays on and the HDD blinks for a sec) The heat sick fan is not spinning but I can hear the HDD running.
I tried the Fn key + power but the only change is that the capital lock/num lk indicators light on for a sec. I also took the laptop apart and refit all the major components (CPU GPU new heat past / Clean of all the dust from fan and other components / Wireless / Ram / keyboard / HDD / DVDrom) but the worst part of it is that I didn't find anything burned fried or smelling like melted hardware.
So as you can imagine I am out of idea what mite be the problem. (P.S. power adapter works and so does the battery)
Thanks in advance if you have any suggestions what for what my next step should be. :-/