Admin Menu Sections Disappeared

First, to rule out any plugin/theme-specific issue, try:

– deactivating ALL (yes all) plugins temporarily to see if this resolves the problem (plugin functions can interfere). If this works, re-activate them individually (one-by-one) to find the problematic plugin(s).
– If you can’t get into your admin dashboard, try resetting the plugins folder by FTP. Sometimes, an apparently inactive plugin can still cause problems. Also remember to deactivate any plugins in the mu-plugins folder (if you have created such folder). The easiest way is to rename that folder to mu-plugins-old.
– If that does not resolve the issue, try switching to the unedited default Twenty Fifteen theme for a moment using the WP dashboard to rule out any theme-specific issue (theme functions can interfere like plugins). If you don’t have access to your admin area, use FTP or whatever file management application your host provides. Navigate to /wp-content/themes/ and switch to the default theme by renaming your current theme’s folder by adding “-old” to the end of the folder name. Alternately, you can remove other themes except the default theme. That will force your site to use it.

Tara, thank you so much! I had not seen your answer until now. I will follow all your instructions.
Kind regards.

Hello, Tara. I already followed all your instructions, step by step, but none of them solved the problem. Could you please give me another possible solution? Believe me: I am getting crazy because of it. I really would appreciate any extra help. Thanks again.

– try MANUAL updating wp-admin folder. Download a fresh copy of the WordPress .zip file to your computer, unzip it, and use that to copy up the wp-admin folder.
– If you haven’t already done, always backup everything (including your database) before doing any actions, just in case something really goes wrong.