If the light is on it means that the modem has an internet connection. It may flash from time to time, this just means that there is internet activity.
If the light is not on then this can be one of the following:
- The username/password is wrong. Sometimes a power surge will cause the modem to reset itself. You can remote onto the modem and check the username/password.
- There is a problem with the account. I see this quite a few times every few years. A client moves and keeps the same phone number also expecting the ADSL service to follow them. Or they were late paying and the service was disconnected.