CPU fan spinning slow, no display, no power on mouse and keyboar

Hi to all, i am a Filipino so please bare with my English and grammars. my computer already not turning on before so i try to replace my HDD then after that i try to turn it on and it is working fine for a few days, one day while i am playing Minecraft (well i play that game most of the time since my computer is working) the monitor just went black also the mouse and key board just turn of but my tower is still powered on led light for power is on led light for HDD stop blinking but it's on, all fans are still on, so i tried to switch is off using the power button but it is not turning off or shutting down the so i tried to force unplug it, tried to plug it again and turn it on same issue no signal and no power to the mouse and keyboard but the computer is on power led is on HDD led is on but not blinking, same on turning it off, can't shut it down using the power button.

I am a basic hardware tech support for dell computers, (BASIC as in BASIC only) so i tried to unplug all the cables a wires i also know how to drain flee power and remove all the static charge on my hand before touching the computer, I also clean all the dust on the computer but still the same issue noting is working.

Lately i try to check if is turning on after a few days letting the computer rest for a while and i notice that the fan for the processor is spinning slower than the usual spin of the fan so i tried to clean the fan disconnect all the power supply clean it then re-connect it then try to turned it on, the processor fan spin faster then it turned on and the computer work fine for 2-3 hours then suddenly it happened again, tried to trouble shoot again same as the issue before computer is on all fans are working processor fan spin slow. My computer is DOOMED again, PLEASE HELP ME, I MISS MY COMPUTER SO MUCH!..