Unchecked all plugins in editor and assests not showing.

From this post: Plugins editor missing? - #4 by Nicat

BrUnO_XaVIeR Engine Contributor

Jan '16

Open your .uproject file with a text editor, there will be a list of all plugins you’ve disabled including the plugin editor one set to load = false…

Set it to = true again and the ‘Plugins…’ menu plus its panel will be visible again next time you open UEditor.