I finally figured it out... its a hardware problem and i'm getting it replaced.. Thanks for all the suggestions
Recently i got my motherboard replaced due to some technical issue and now the networking doesn't work... i Tried re installing the drivers but nothing works... What should i do now ??? BLUETOOTH WORKS FINE DETAILS: Dell inspiron N5420 laptop OS: Win 7 Ultimate 64 bit Qualcomm ethernet drivers 
Networking doesn't work ! Help me ASAP !!