Read here: Fix for black screen when launching Valheim.
It's been confirmed now that this works for the users who have the issue.
in your player.log file in C:\Users\ <username \AppData\LocalLow\IronGate\Valheim
(or linux wherever you put files of your choice)
If you have these outputs:
"ArgumentException: RenderTextureDesc width must be greater than zero.
Parameter name: desc.width"
You will launch with basicly everthing graphical being 0 size...
To fix this, add "-screen-height 1080 -screen-width 1920" as shown in the picture to your Launch option steam.
Then when you get into the game, immediately go to <settings> and choose a screen resolution and then in MISC. enlarge the UI setting.
Then go back out again and select character and world. play for a few seconds and then press Escape key and pick <Quit>
This saves all your settings. Then go back to the Launch options and clear then remove "-screen-height 1080 -screen-width 1920".
Then launch the game again to see that it worked :)