How do you diagnose and repair a motherboard failure?

Having a good flashlight for examining parts on the motherboard, like RAM & expansion slots often will help you find debris, defects, or damage in the slots. Also it’s extremely useful for pinpointing bent (or slightly out of alignment) CPU socket pins. One thing that can easily go unnoticed and be a sign of damage is a board that a slight warp or bend to it. Most boards have a tiny bit of flex to them but anything that doesn’t sit mostly flat on a table can be a red flag of damage to the internal traces inside the board PCB.

My opinion is similar to Kenneth's regarding "it's way more cost effective to replace the motherboard than to spend the time to diagnose individual components and then replace them." Myself, I would scratch out the "replace the motherboard" part and put in TO BUY A NEW COMPUTER then to spend the time to diagnose individual components and then replace them. Unless it is a specialty system or an expensive gaming rig, buying a new computer altogether is generally a CHEAPER option. The failed MoBo could have damaged other components too so there's a risk with that as well.

If we're talking about a cheap/oem system that is ready for retirement, then sure: replacing it probably makes more sense than repair. Given that our e-waste situation has nearly reached epidemic proportions, I'm always thinking of product lifecycles and how we can continue to use otherwise throw-away tech. Example: government attaches typically have a 3yr replacement cycle for PC's and laptop computers. They are normally still good, but may not meet the technological needs (of that specific agency). By finding ways to repurpose tech, we can actually (r)educe demand for new systems and (r)euse tech we have already made. That's 2 out of the 3 R's in recycling, people, and no small thing!

Just my notes on motherboard failures. In 35+ years of PC repair, I've seen only two instances of CPU failure. One was a Cyrix and the other was a slot 1 Intel CPU. I disagree that replacing the motherboard is more expensive than repairing it. Other than some specialty systems, it's way more cost effective to replace the motherboard than to spend the time to diagnose individual components and then replace them, especially with the equipment, time and expertise needed to do repair work on modern motherboards. The only exception would be in the obvious case of a bad power section with bulging capacitors or burned power transistors. Those can be relatively easy to diagnose and replace.

The very first thing is to check the fuse in the power cord / plug as a blown fuse will not allow the PC to switch on, it’s worth also checking the PSU as often these have an on/off switch which can be knocked by accident but also check the power cable is firmly pushed into the unit itself.

Strongly consider whether it is worth your time to repair motherboard components. Unless it is something quick and easy like a CMOS battery you're usually better off with replacing the component.