WordPress 5.5 changes the way JQuery works, essentially it removes some old deprecated code from WordPress with the view of speeding up user experience. However, some older plugins cause JQuery to crash, which then effects every other plugin on the site.
This can lead to weird effects like drop down menus no longer working, or sliders no longer sliding, or plugins like WP Bakery Page Builder not loading on admin pages.
A workaround solution is to install and activate the “Enable Jquery Migrate Helper” plugin. This restores the JQuery functionality to what it was before WP 5.5. It will also explain at the top of the Admin panel which Plugins are creating JQuery problems. Hopefully those plugins have updates available, when they do, upgrade them and try disabling the Jquery plugin to see if your site works correctly again.