Using LibreOffice 6. Working in Writer.
I assume you closed your session. This is too late, everything has been written to disk. If, by chance, you have a backup, use it to reenter your latest changes. Otherwise, you may try some salvage utility (various apps depending on OS which you didn’t mention) which may recover with a lot of chance an older version of the file, but don’t count on it. The odds are a factor of the file system technology (mainly whether it is journalled or not), total percent of disk used (on a full disk, deleted space is reused quickly), etc.
In case you asked in emergency with your documents still open, just save the long one to restore the situation.
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This has worked for me, but I have L. O. backing up my files, not sure if I did it or if it was by default. To find the path where there saved, in Writer, pull down Tools/Options/Path. Backups shows path to the old, deleted, name changed files. In my file manager I went to that path, copied my deleted file to “Download”, renamed the extension so L. O. would open it, then open L. O. and went to “Download” and it opened up my deleted and old renamed files. If it’s not there, you may want to ask L. O. to backup your files from now on, Tools/Options/Load/Save/General. In my “General”, all my boxes are checked, and I ask it to backup every 30 minutes. Good Luck.