Please double check to make sure the line settings are still there in Express Talk (even if you did not change them, it has been known that sometimes the settings are not saved).
If the settings are there, are you sure they are correct. If Express Talk is reporting a "Server is not replying" error, then it indicates to me either the VoIP settings are not correct, are no longer valid, or the VoIP service is experiencing technical difficulties.. OR something has changed with your connection to the internet and Express Talk is unable to properly communicate with the VoIP service.
It is highly unlikely registering the product has caused this problem. If anything the fact it stopped working is a coincidence, or it is possible that after the upgrade some of the settings were reset and need to be set correctly again.