MacBook not booting after update; fan spins, board heats up, no chime

So the basic symptoms are the following, way more details below:

• Macbook holds charge; charger light solid orange until green when fully charged
• When I press the power button nothing happens, except the fan turns on and board begins to heat up. Nothing on the screen, no chime, no backlight, nothing when I press the caps lock key. I only know the fan turns on because I opened it up, it’s not loud.
• The fan stays running and the board continues to heat up until I hold down the power button to turn it off or the battery runs out of power.

Extra details:

• Laptop was left in closet for a few months before turning on. Turned on fine, began installing OS update, then shut down and wouldn’t turn on.
• A few months later, tried turning on again. Didn’t fully turn on, just kept hearing the chime sound looping, but without ever sounding completely. In other words I pressed the power button once, the chime sound would play repeatedly without the screen ever turning on.
• After this, tried SMC reset, and it actually worked. Computer came on was able to login and use as normal. I remember installing a program, a demo for some software. Don’t remember at what point I turned it off, but I guess I did or it ran out of charge.
• A couple weeks later, tried turning it on again and arrive at current issue, described above.
• Tried NVRAM reset, SMC reset (which was successful based on charger light indicator), and same with battery disconnected and the charger plugged in, but the macbook still doesn’t turn on. Tried reseating screen cable, no change either.
• I tapped around the board with my fingers, the parts that get hot are throughout the board, especially over (what I think is) the CPU.
• I tried this other thing: I held down the power button without the battery plugged in to drain any residual power, then plugged in the charger while still holding the power button. Held for a little while longer, then let go, then pressed the power button again. The fan starts up and goes really fast but nothing else happens. If I do another SMC reset it goes back to the state described above.
• The board is a little dusty

What could the issue be? Anything else I can do or check? I’ll even go through a troubleshooting manual if I there’s any available, don’t mind learning a bit and messing around with the insides. Any resources would be appreciated.