The previous answer assumed some knowledge on the part of the user. This one adds some elaboration.
When an instance of LibreOffice is running a hidden file is set. This hidden file is called ".lock", the "." indicates it is normally hidden from view.
When you open a terminal window it will normally start in your home directory and will only show you files that have not been set to be hidden. The home directory is the directory you "own" and normally operate within unless, of course, you assume the persona of the administrator or another user. So if you type cd .config/libreoffice/3 it will take you to a directory folder, described by "~/.config/libreoffice/3"; the "~" indicating to the operating system you are referencing this command from the point of your home directory.
When you arrive in the folder "3" you will not be able to see the ".lock" file by typing a simple "ls" because, just like the ".config" folder, the ".lock" file is hidden from view. To see it, type "ls -al". This will reveal any hidden files in the listing.
To remove this file, once in the "3" folder, you type "rm .lock". After that you might like to type ls -al just to be sure the ".lock" file is no longer present.