I tried turning on my raspberry pi 3 b+ and it wouldn't show anything to my TV, the entire Pi ( mainly around the SD card ) was so hot I could barely touch it. Bare in mind the pi was only on for about 1 minute. Could it have been shorted?
There is a chance that some connections in your rpi is shorted somehow. It seems your board is not working again so I am leaving some suggestions for others. Always clean your raspberry pi with a brush. The main problem of unused raspberry pi is they have dust on them and conductor particles can easily burn everything if not cleaned properly.