It does sound like a motherboard fault, likely a short circuit has occurred somewhere. It can be something else though, like bad RAM, drives or another thing connected to the motherboard. Have you tried removing the drives you have connected (HDDs, SSDs)? Have you tried resetting the CMOS? If you can find a CR2032 battery, take it out and leave it for about a minute. Put the battery back in and try to power up. Give the laptop some time as it may power cycle and reboot a bit before actually POSTing. When you reset the CMOS, make sure there's no power source connected (battery and charger). Only connect a power source once you've finished. As this seems to be a power-related fault, what you can do is remove some connections such as fans, speakers, audio boards etc. Leave screen connected for now. If you can observe the same problems with either nothing or very little connected to the motherboard, chances are you have a motherboard fault. If that's the case, the easiest thing is to replace the motherboard. Troubleshooting circuit board issues is complex and requires some tools and experience. If you can't find a new motherboard that's reasonably priced, then the best bet is to replace the laptop. As a last resort and if it's easy to do so, disconnect the laptop screen from the motherboard. Insert a stick of RAM (if you have removable RAM), ensure the power button is connected (it'll either be part of the keyboard or you'll see a separate power button circuit board that connects to the main motherboard) and then connect an external screen to the laptop. If you can't get anything from the external screen, then you can rule out a bad screen issue. Before you remove the laptop screen, make sure there's no power source. Ensure battery and charger have been disconnected. Never disconnect/connect a laptop screen with either power sources still present. It can cause further issues.
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