Safe mode is a mode where LibreOffice temporarily starts with a fresh user profile and disables hardware acceleration. It helps to restore a non-working LibreOffice instance.
Choose Help - Restart in Safe Mode.
Start LibreOffice from command line with --safe-mode option
Once in safe mode, you will be shown a dialog offering three user profile restoration options
This option will let you work with LibreOffice as you are used to, but using a temporary user profile. It also means that all configuration changes made to the temporary user profile will be lost after restart.
Choosing Restart in Normal Mode will discard all changes, terminate safe mode and start LibreOffice again in normal mode. Use this option if you got here by accident.
The dialog offers multiple changes to the user profile that can be made to help restoring LibreOffice to working state. They get more radical from top down so you should try them successively one after another. Choosing this option applies selected changes
LibreOffice keeps backups of previous configurations and activated extensions. Use this option to return to the previous state if your problems are likely to be caused by recent changes to configuration or extensions.
You can disable all extensions installed by the user. You can also disable hardware acceleration. Activate this option if you experience startup crashes or visual glitches, they are often related to hardware acceleration.
Sometimes LibreOffice cannot be started due to extensions blocking or crashing. This option allows you to disable all extensions installed by the user as well as shared and bundled extensions. Uninstalling shared and bundled extensions should be used with caution. It will only work if you have the necessary system access rights.
If all else fails, you can reset your user profile to the factory default. The first option Reset settings and user customizations resets all configuration and UI changes, but keeps things like your personal dictionary, templates etc. The second option will reset your entire profile to the state when you first installed LibreOffice.
If you could not resolve your problem by using safe mode, click on Advanced expander. You will find instructions how to get further help there.
If you want to report a problem with your user profile, by clicking on Create Zip Archive from User Profile you can generate a zip file which can be uploaded to the bug tracking system to be investigated by the developers.
Be aware that the uploaded profile might contain sensitive information, such as your personal dictionary, settings and installed extensions.