Ok, so I just dealt with this today. Over one thousand hours. Gone. I was very, very sad . IDK what happened, I think it had to do with my cloud save when I tried starting it up on the Steam Deck? It was out of sync, and I must have just chosen the wrong option.

Anyways, I spent a lot of time today and just couldn't get it going. I thought of giving it another shot, and I now have everything back. What I did was:

1. Download the backup files from cloud storage . You have to be signed into steam in your browser.
   
2. Look for Valheim, look to the right, and click the 'show files' button.
   
3. Look for a database (.db) and .fwl file that includes "backup" in the name and download them.
   
   Should be formatted something like this (mines gonna show different dates/times):
   "IronGate/Valheim/worlds/Myworld_backup_20220627-195041.fwl"
   &
   "IronGate/Valheim/worlds/Myworld_backup_20220627-195041.db".
   
   
4. In windows file explorer go to "C:\Users\"your name here"\AppData\LocalLow\IronGate\Valheim". You should find worlds folders; 'worlds' & 'worlds_local' make a copy of these folders and save them somewhere like your desktop or whatever just to be safe and have a backup in case things go awry.
   
5. Delete all of that worlds files in both world folders (not the copy you just made). Just delete the files for the world you're trying to restore, leave other worlds you may have created alone. Do this for both worlds folders.
   
6. Take those .db and .fwl files you downloaded and rename them to whatever your worlds name is but be sure they remain a .db file and a .fwl file and paste them into both of those 'worlds' folders you found here: "C:\Users\"your name here"\AppData\LocalLow\IronGate\Valheim"
   
7. Then hold onto your butt and start the game. You should have everything back.

I hope this works for you. Everything I read online and saw videos about didn't seem to mention the files including 'backup' in the name but figured to try and it worked for me. Good luck!