Very occasionaly I ran into the problem of losing network connection after using Fiddler for debugging with web traffic. To be more precise, this problem usually happens after closing the Fiddler application, and it seems more reporducible when using the FiddlerCore API(However, this is very likely because my code didn't release Fiddler in the correct way...) After this problem happens, and when I open IE, I see the error message of "The proxy server isn't responding" as follow:
This is caused by, as far as I see it, that Fiddler sometimes fails to unregister itself as the system proxy upon on quitting. In that case you will be redirecting all your HTTP traffic to a proxy that doesn't exist, and thus gives you the error shown above.
Easy way to fix this, is to reluanch Fiddler application which will act as system proxy again (make sure you set the option in "Fiddler Options" - "WinINET Connetions"). Close the application gracefully and it should fix the connetion path for you.
Or, you could manually clear the proxy settings by going to Internet Options, select "LAN setting" in "Connections" tab, and uncheck the option of "Use a proxy server for your LAN". (In which you should see a proxy like 127.0.0.1:8888, which is the default listening port for Fiddler.)