Hide Joomla Main Menu
If you do not want to see any Menus on your screen, that will of course mean the Main Menu as well. However, do make sure that you do not delete or 'Trash' the main menu as it contains the 'Home' file which provides the configuration details for the front page formatting for the website. In fact removal will mean that the site becomes inaccessible and any attempt to get in from a browser using the main website URL will mean a 404 page being displayed.
If you do not wish to see the Main Menu on your website, try using the following setting on your Joomla website:
Extensions>Module Manager>'select' Main Menu>Menu Assignment>'select' None
...and of course, reverse the setting to get it to reappear again.
If you do remove the Main Menu, there are workarounds. However depending on how advanced you are with your website design you may feel that the easiest thing to do is re-instate the previous version of the MySQL database - providing of course you've downloaded a backup copy - or you may even feel it's easier to reinstall Joomla.