This is a tip on how to solve the issue in the title. This can occur to you when you open a page for editing and focus your mouse on the main content area. Normally the content editor should become active in the ribbon but this may not happen or you just can not add content.
The solution for this is to add the following URL postfix after you aspx page ?contents=1.
Next you should be able to select the webparts which you know are in your content area. Close those selected webparts (you may need to check out the page). After this go back to your page and add new content. When you are ready then focus your mouse in the content area to add a new webpart. Your webpart should be under a category named something like “Closed webparts”. Add the webpart and everything should be OK. The webpart properties and state should have stayed as it where before it was closed.