Even though at this point I suspect the mobo, the very first thing to check is the PSU. Even though everything appears to be getting power, something may not be getting the right voltage. And that's something you can't determine just by looking at lights and listening to motors.
Warning! Before installing or removing any component in a PC, the power plug should be disconnected from the AC mains, and you should wait 5 minutes before taking further action. Some people further recommend after pulling the AC power plug, to then push the computer's front-panel power switch to bleed-off any residual power in the PSU.
And of course, proper ESD (Electro-Static Discharge) procedures must be followed.
Not following these procedures can cause permanent damage to the computer and components.
If you have a go/no-go PSU tester, use it. If you know how to use a volt-meter, use that
also. Visually and electrically test
all the connectors. Those simple testers won't identify all possible problems, but they may find the basic problems such as no voltage, bad connector, out-of-range voltage, etc. If you have
only a volt-meter, remember that the PSU requires a minimum power-load in order to function properly and provide accurate readings. Remember also that a typical volt-meter only reports
average values. Only the more expensive meters will let you check for voltage transients. out-of-specification ripple or peak voltage, etc...which can cause a computer to malfunction.
If the PSU "checks out OK", then...
The next thing to do to see if you can get a screen to display in the
absolute minimum functional configuration:
Remove or disconnect
ALL devices except PSU, mobo,
ONE stick of RAM, and the display. Yes, that includes disconnecting the keyboard and mouse.
If you have integrated video, remove the video card and use the integrated video connector. Otherwise, leave the video card installed and connected to the display. But, keep in mind that a bad/mis-seated video card can cause a failure including a failure where the PC won't display anything, and won't post any error codes or beeps.
While you're at this point, you might as well clean the system vents, heatsinks, fan-blades, etc...all the "stuff" related to cooling.
Inspect and re-seat the device connectors, including power connectors.
Perform the motherboard/BIOS settings "reset" procedure. Usually involves a jumper or removing the battery. Your manual will tell you.
Boot the system. It should at least start showing POST (Power-On-Self-Test) progress.
If it didn't, and you have more RAM sticks, try removing the current RAM stick and using each remaining stick,
one at a time to see if you can get the system to get to POST with just one RAM stick installed.
If you can't get the system to at least display something at this point, then you know it's a fundamental problem involving those core elements...and that needs to be addressed before any more debugging can be attempted. But, at least the problem is isolated to one or more of these few devices.
edit-add warning note regarding pulling AC mains power cord and following ESD procedures
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