Yup some details would be good... do you get the line sync light on your router flashing or lit?
Something to try is ask your isp for some test usernames/passwords so you can check that it is your connection or your router and not an incorrect username/password, for example i know of a "bt_test" one.
If the router is only 4 months old is it not covered by a 12 month warranty from the manufacturer?
