I noticed this issue today, and I wanted to point it out for anyone who might have seen a similar issue. When Fiddler starts up, it sets your LAN settings check box to use a proxy server for your LAN, and when it closes normally it unchecks the box. However, yesterday my computer shut down without closing all applications manually, so Fiddler was forced closed, and when this occurs the check box is not unchecked, and you are unable to use the internet because your browsers are attempting to connect to a proxy server that doesn't exist. It's a quick fix once you realize what's going on, you can either start Fiddler back up, or go into your internet properties -> Connections -> LAN Settings and uncheck the box manually.