We recently had a steam deck in that wouldn't display, both on the internal screen and external screen, the information to fix both issues was scattered around the internet, hidden in comment threads so we have decided to make this post to hopefully speed it up for others.
In order to fix the no internal screen issue we first need to get the external display working, for us this was simple all we had to do was to reset the bios which is holding the three dots ... button below the right track pad and volume down button and press the power button.
if this does not work, then do what we actually did, try it around 10 times, power cycling it, charging it, sleeping on it and then if your lucky like us the next day it will work first try!
Hopefully you have external display working now, as this next part is a little more involved.
Step 1 - Go into settings and enable developer mode
Step 2 - Enable 'Show Advanced update Channels' in developer settings
Step 3 - Set OS Update to 'Main'
Step 4 - Apply software update and restart
Step 5 - Force Power Off (Hold Power 15+ seconds) then you can unplug your external display
Step 6 - Use Recovery mode, hold Vol minus and the three dots ..., while holding them, tap the power button
Step 7 - we plugged in the power cable at this stage and then got a flashing white power light, then all you need to do is wait, around 15 minutes and your screen will pop back on or at least ours did, Good Luck
