Sorry to hear your having issues…
What theme are you using? Did you make any change to original code, before the update? Did you backup your theme folder before updating? Did you go through all the settings (plugins, etc) and see if any of the settings have changed, sometimes major updates will reset everything to default. Is the plugin driving the menu, up-to-date?
In the future before updating plugin or a theme, log in via FTP to your server, copy the theme folder or specific plugin folder, and rename it to
jquery-mega-menu-2. This way when you update, if somethings wrong, you can just delete the original (one with the update,
example-theme) and just rename the backup you created
example-theme-2 back to
example-theme, restoring it to previous version.