Not sure if it can be salvaged, but you can try this:
- Go to this folder:
C:\Users\<your user name>\Documents\My Games\Terraria\Worlds
Your world file should be in here, and there should also be a duplicate file, except it has a .bak extension.
So you have a file named <your world>.wld and a file named <your world>.wld.bak.
If you can't see file extensions, click View in the file explorer, and enable "File Name Extensions" (Windows 10), or click View -> Folder Options and find Enable File Name Extensions somewhere in this window (previous Windows versions).
Now, read this well:
- Remove the world file that has no .bak extension (just put it into your recycle bin)
- ReNAME the world file with the .bak extension to <world name>.wld.
Try if it loads.
If it doesn't, or if you don't have a .bak file here, then I'm afraid your world is lost.
I recommend making manual backups once in a while.