Proxy Authentication error, internet stopped working

I keep on receiving a popup box saying:

Proxy Authentication
Your proxy server requires a username and password. Contact your network administrator for the correct settings and supply your proxy username and password in the "Connection section of Settings.
Proxy server: 0.0.0.0:0
Username:
Password:

The internet is not working, and I'm not using a proxy, so I don't know why this message keeps on showing up even after reboots and making new users.