Lets say you opened a document library from you browser, clicking new word document, and trying to save it to the library.
but MS Word is not showing the document library
and even if you entered the URL in the address bar its not working
This is because the WebClient service is turned off, so your PC cannot communicate to SharePoint using DAVWWW -also it breaks the explorer view-.
Turn the service on (if its disabled, change the start type to automatic) , close MS Word and try again, everything will work properly.