|There are numerous possible causes for your situation and it is a process of elimination to work out which. |
Q? Have you ever opened a computer case? Do you know the basics of computer hardware configuration? Is your machine still covered by warranty?
Open the case. With power disconnected, remove and re-seat all power & data cables. Simply unplug and re plug them from motherboard/drives etc one at a time to make sure each is fully seated. This process can also clean any slight corrosion from contacts. Clean out accumulated dust using canned air.
Now connect power and try to start up. Any sign of movement from fans?
Common cause of this problem is powersupply failure. Do you have access to a spare compatible powersupply of equal or greater capacity? PSUs can suffer partial failure. Motherboard LEDs can still be lit while other parts of PSU have failed. Without a known good powersupply you cannot test the other components to see which if any other things have failed.
Your problem may be as simple as a failed CPU fan or a faulty drive up to a failed CPU or motherboard.
Goin' Fishin' (Some day)