@zep tepi is right, here's a bit more info ...
The CHARACTER files are saved seperately from the WORLD files.
Both are located here on Windows 10:
C:\Users\{YOUR USER NAME}\AppData\LocalLow\IronGate\Valheim

Characters and worlds are separate folders.
In the WORLD folder there are two files that make up the "world" -
{NAME OF YOUR WORLD GAME}.fwl and {NAME OF YOUR WORLD GAME}.db
Like for mine - Weingrad.fwl and Weingrad.db

For the CHARACTER it's one file - {YOUR CHARACTER NAME}.fch
for example mine is the rock.fch

When the game closes these files are updated, so if you open a game and see ti's broken DON'T EXIT THE GAME! Instead hit the Windows Key or use Ctrl + Tab to switch out of the game (easier with two monitors...) and go to the folder (I have a shortcut on my desktop but I'm weird). You can then compy and paste the files for safety.

Each time it exits it ALSO makes backups of these files (both World and Character). Like:
Weingrad.fwl.old
Weingrad.db.old
the rock.fch.old

If the game is RUNNING the normal files (not the .old ones) are the world BEFORE you opened the game. The .old are the world saved before that, hopefully still good.

To make sure it's clear, the three files are updated and the last session is renamed
blah-blah.blah.old so there's three files and three ".old" which are the last session. That's the Devs backup plan.

BUT..!

If you open, find a problem (OH S**T!), exit, RE-OPEN and exit AGAIN, you're screwed because the files have BOTH been updated; the "normal" and the "old" since you exited twice. {SIGH}

There is one SMALL possibility of safety (assuming like most people you don't run a sytem backup daily). Windows has a "Previous Files Version". IF it's setup on your machine.

If you RIGHT click a folder you MIGHT see an item "Restore previous versions". If you get that, AND your system happened to take a snapshot, you'll see previously dated folders. You can restore one of those from before the problem ( you have to do both charcter and worlds folders) and you'll be restored to that date / time. Might lose a few days but better than a complete loss, eh? IF it works.

Doubt this is timely enough to help but I tried. :-)

One last thing to help in the future - I just posted this in another thread (i'm crossposting to save time) :

I run a "Mod" (not really, it's OUTSIDE the game, just a Windows Batch File) called Backupheim that does a backup of the World, the Character, or everything. Great for cases like this where you can lose everything! I LOVE Valheim, but IMO the "backup" system is a total disaster.

Here's what I use: https://www.nexusmods.com/valheim/mods/1325

I actually wrote my own before I found this, but his is more polished ... ;-)

Use tip: You can run it when the game is running but it WON'T have the proper CURRENT data as Valheim only updates the WORLD files on EXIT or when an Autobackup occurs. The CHARACTER is only backed up on EXIT or LOGOUT. (You can use the F5 key console to force a World save, but it DOESN'T get the Character. Crazy. BROKEN. My opinon.)
So run this every time you exit or have accomplished a major task (killed a Boss, etc), certainly once a day. You then have mutiple backups of everything.

Cheers and good luck!