Suddenly, my character is no longer appearing when I launch Grim Dawn. My save is still there under: C:\Users\<username>\Documents\My Games\Grim Dawn\save\main\_<charname> but it does not appear in game. Any ideas?

Maybe check the folder properties. Is it read-only?

Maybe you made hardcore character and died?
Try GD defiler, it usually can repair damaged saves

The save game appears fine on my laptop, but not my desktop, even if I copy the files over manually...