It seems like your Default server document has disappeared, somehow.
You could try to add it back.
In the bottom left corner of your Configuration Editor, there is a
"Server Status" window with a "Add Server" button.
Click that button, and specify the name "Default".
Verify that all the parameters are what they should be, correct if needed.
Save the document if you make any changes.
Now there should be a "Default" server in the "Server Status" window.
Select that, and click the "Start" button above it.
Maybe that will help?