I had a 200,000 word document that I tried to justify. I pressed the justify button, and all my data was eliminated. What to do?

How can I retrieve a lost document?

I had a 200,000 word document

… and no backup? If you did not take an precautions by yourself and you already closed the app, there is no way to retrieve a lost document. If you are a lucky guy you may have set Tools -> Options -> Load/Save -> General -> [x] Always create a backup copy once in the past and you may find the file in directory defined by Tools -> Options -> Paths -> Type: Backups (as <name_without_ext>.bak)

The first thing to have done immediately was to have pressed Ctrl+Z (undo), everything would have returned to normal.

If images didn’t return properly, you could have closed without saving and then opened the file again, you would have lost anything since the last save but probably only a few minutes work.

You next options are as @anon73440385 says.

You future course of action is to learn how to use styles. You want to justify just the Text Body style by right clicking it in the sidebar and selecting Modify, adjust the alignment in that dialog. That way you don’t need to select anything to change alignment. Your headings won’t be justified either which would look very odd

There is a menu entry, Save a copy, that you might utilise in the future. Each day, or before you make major changes click File > Save a copy, append the name with date (and maybe time) in the format YYYY-MM-DD (and HH). That way you will have backups that you can fall back to. After a while you can delete the earlier ones.

Save working copy and backups to a second location outside the current computer

In addition to @EarnestAl

Preventing data disaster