|Nothing you have said gives any assurance that your CPU is actually working.|
The fact your CPU fan and other fans spin up is no guarantee of anything, not even that the powersupply is fully functional.
Before the problem began did you usually get one or two bios beeps during startup?
This kind of troubleshooting has been described many times.
There is a fairly simple test you can do. With power disconnected remove all your ram. Disconnect power & data cables to all drives, and pull any PCI cards. Now connect power and try to start up. If you get bios beeps then you know a couple of things. The powersupply is probably working. The CPU is functioning since the BIOS relies on it. If you get no beeps then you are left with three possibilities.
1. The powersupply has partially failed.
2. The CPU has failed
3. The motherboard has failed.
Or a combination of 1,2,&3.
If you do get bios beeps with all ram removed then re-insert 1 stick and try again.
Do you have onboard graphics or a plug-in graphics card?
If a separate card, remove it and try booting.
If you get bios beeps with the card removed but no display with it in place then there's a good chance the graphics card has failed.
Unfortunately the only sure way to identify which part has failed is to swap in/out known good compatible parts, beginning with powersupply
Goin' Fishin' (Some day)