It could be a problem at either your phone jack or between the wiring from your phone jack to your NID box outside.
Try taking your dsl modem outside if possible and plug it directly into your NID's phone jack and see if your modem connects to the internet then.
If it does then it's probably bad wiring from your NID box to your phone jack or could be the phone jack itself.
If it still doesn't work then your dsl modem might be bad or it could be something to do with the internet service providers network.