Internet connection problem

Hello,
i'm having a problem with my internet connection. i'm using BSNL Broadband connection through a seimens modem. My net was working properly till a month back. but, suddenly it stopped wurking. now, it validates the username and password, n even shows the status as connected. but, i cannot open any site on either mozilla firefox or internet explorer. but, the same modem is wurking properly on a different computer. please help me out.