Just had the same thing happen to me. I was trying to start a new "advanced" world with stuff robbed from a previous one. After about 5 hours, while it was saving, it just stop responding. I force closed Valheim and started again. Day One with no gear/items but the fog on the map was gone like i had went places.

Luckily, I have many worlds so no big loss. But it is a minor annoyance.