Few days before I got a problem in my bsnl broadband connection.Yes Modem lights are on and System Network Connection status connected but when I open a site in browser website is not loading then I went to bsnl office they did small changes in my modem configuration.
He opened admin panel http://192.168.1.1 and he went to WAN setting then he entered my username and password(given by bsnl).After rebooting my modem in same admin panel that problem got solved.
I am using Nokia Siemens modems.Above Steps may different for different modems.I think this step is called configuring modem setting.
I think this problem is mostly coming when you upgrading or reinstalling operating system(Not Sure).