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