Did you have mods installed? They're probably still there and break the game.

What does C:\Users\<YourWindowsUserName>\AppData\LocalLow\IronGate\Valheim\Player.log contain any hints? Does it contain any information about the recent starts?

Take a look in any ProcessExplorer/TaskManager. Are there more than one (hung) instance of valheim.exe? etc....

Hardware OK? Memtest86+, SMART Data, Windows system logs/protocols, other steam games are working fine?

Add
in VHs Steam game options as a launch parameter, try again and look at what the console tells you. + Take a look at the log again.
Does the console even show (not the ingame one but the cmd-shell one logging what VH currently does)?


When you don't try to investigate yourself first and tell us what you've already tried its impossible to help because it's just fishing in the dark.

Narrowing down possible reasons from near infinity (="it doesn't start") isn't possible.