If a former state was lost, it is likely that the game was saving on the Cloud, and not locally, so the folder to peer at should be 'worlds' under Valheim, instead.
Also don't forget to reload the character saved at the same time (here "Charactername.old" in 'characters' folder).

On the "Select Worlds" dialog (after you select your character) is a button to manage saves. Each world expands to a list of backups. It does not show dates, but your most recent backup should be "Worldname.old."

I'm running with Steam cloud saves disabled. It seems every time this sort of thing happens, it's a problem with cloud synchronicity.

If you'd rather use your file explorer, there is a button on the lower left of the opening screen which opens the ...Irongate/Valheim/ directory. (The caption is "Show log file," but that's what it does.) Your local data should be in worlds_local under Valheim.

hth

I'n not gonna give you step-by-step how to do it but pretty sure world saves have backup files. I'd convert the most recent backup into the normal save file.

Look up how to do that or maybe some kind soul will walk you through it.