I would unplug both the ethernet and usb cable from the modem and power it off. Shutdown your computer at this point also.
Wait for about a minute (longer if you get sidetracked making a brew or something
)
Plug the ethernet cable in (I assume you are using the ethernet cable that the NTL engineer left behind?) to both the modem and your PC.
Power the modem on. Wait for the lights to settle (this can take a moment - good things come to those who wait!)
Power your PC on. Try the internet when it's booted up - should be working no problems.
You may need to go through the computer provision system again... If you try going to ntlworld.com - it should detect your IP is one used for registration and forward you to the autoreg site. If not, go to
http://autoreg.autoregister.net and follow the instructions onscreen. You will need your PID code and your account password to continue by the way.