I have used Emacs Org mode for my notes for several years, and I use Prelude configured with evil. Everything works fine. Until a few minutes ago, I found the note file is empty of content. and the file size change to 2 bytes. I try to undo but it's useless(I get nothing through undo). How can it be fixed?
In a shell window, make a backup of your files (now), including the autosave files.
After backup. You can probably copy the autosave file over the empty file.
However, and this is very important, make a backup, before you start trying to fix it.
Then when all is good (or not), you need to create a backup strategy, for all of your important data.