What would cause DSL light to be on and internet light to be off? DSL on & internet Light off.

 Binatone DT845W modem

During last September 2012 , I have bought a Broadband  ADSL + wifi modem of Binatone compnay. The model no of my modem is Binatone DT845W and got it configured with BSNL Broad band connection. For two three days, it worked very fine but after that frequently its internet light got off sometime due to sudden power failure or sometimes without any reason. I got fed up with as I several times called BSNL people, they come and check their phone line and broadband setting and all the times they found everything fine but even then my modem internet light is not On, I did many R&D on my modem like when I repeatedly make power supply switch on-off, on-off it get working some times not.