Disappearing CiviCRM menu and Network Error
This error typically means there's a JavaScript error.  Open your Developer tab in your browser (by pressing F12), go to the Network tab and reload the page.  Consider filtering by "XHR" requests, since it's almost certainly a problem there.  Look for the one with a status that's an error and click on it, then check the "Response" tab.  You'll find a more detailed error there.  The error will probably be more detailed if you enable Debugging and Backtrace.  Hopefully that should be enough to solve your problem, or at least update your question with more details!