MBP 13” 2020 won’t turn on after 11.3 Beta 3 Update I left my laptop downloading the update because it was taking sometime. After an hour, I saw my laptop’s screen was black. So I tried powering it up, but nothing happened. My laptop was not drained, because I left it with the power cable attached to it. My MBP is showing no signs of life. I have done all the things apple support was asking me to do. SMC Reset, Recovery mode, discharging everything, but there was really no response from the power button. Has anyone experienced this? And did you manage to fix it? This is so frustrating. I just wanted to update my laptop. 
PS. I tried holding the power button for at least thirty seconds, and the keyboard starts to light up a little bit. But if I dont press anything for a continuous time, it dies again.
A few things I would suggest trying.
Since you were installing a new update that is in BETA (11.3 Big Sur) this runs the risk in many factors but nothing to fear...yet.
Let's try a few things first before seeing if your T2 Chip got a corrupted Firmware patch.
You mentioned your MBP is turning on but no screen is coming up.  I would suggest unplugging your MBP first.
Another suggestion is to see if you can spend some money on an external USB C tester here -&gt; USB C Tester to see what Voltage and Amps you are getting.  When plugged in with a USB C tester a MBP 13&quot; 2020 will give you 20.0 Volts and Amps around 0.4 and climbing to a steady 1.09 and above.  This means the computer is turning on etc.  With the tester you can verify if the computer is boot looping.  What this means it is MOST certain it is a Firmware patch issue, WHICH can be resolved with DFU mode.
Now in order to do DFU mode you will need another MacBook that has a USB C connection that has the same OS as the one you currently tried to install on your computer.  BIG SUR.  I suggest using any MacBook Pro from 2016 and above.  You will also need to install Apple Configurator 2 from the App Store on the Host computer.
There is a YouTube guide on how to do all this. But keep in mind you WILL need a MacBook with a USB C port.  Here is a link to the DFU guide for your MBP.
I am certain you corrupted your firmware and you need to repair it.  (Apple will say it is a logic board issue, which it is but it is something that can be repaired with DFU mode and no need to pay Apple for it.)
Here is the YouTube Video how to setup DFU
https://www.youtube.com/watch?v=UaVgBP4gJsU
Here is the source to learn more about how to repair in DFU step by step:
https://mrmacintosh.com/how-to-restore-bridgeos-on-a-t2-mac-how-to-put-a-mac-into-dfu-mode/