So… I can’t necessarily fix your problem. I can help you fix the problem if it happens again.
Option 1: File Recovery Plugin
This plugin is one of Obsidian’s core plugins. See if it’s enabled.
If so, go to
File Recovery under
Plugin Options, click
View Snapshots, search for your file, pick the latest version from before you merged the file, copy the contents to your clipboard, and paste it into your file to replace the merged content.
Option 2: Git (or another VCS)
Because this file is a day planner, a VCS (Version Control System) may not be the best solution, as you’d have had to run the VCS prior to merging the files but after you wrote useful content to that file. I have a git repository of my vault, and every night (at least when I remember) I run a git commit of all files in my vault. If I ever need to know what a file looked like on a given day, I can go to that commit and see, keeping it if need be.
Option 3: OneDrive
Is your vault in your OneDrive? If so, you can recover past versions of the file through onedrive sync.