Describe as much as you can how you launch the game, and what happens.
Which is your Processor, Graphical cards, RAM memory?
Have you any mods installed? If so, please uninstall, verify your installed files (worst cate, reinstall have worked for a few players)
Have you changed boot.ini file?
Have you steam version (Build mars 23 2021) if you don't, restart steam or your whole computer)
Try to disable steam cloud saves (picture attached)
Some files are saved in C:\Users\ <username \AppData\LocalLow\IronGate\Valheim backup your world and character folders and remove the rest.