mate, think people will need to get the make/model of the modem before they can help any further.
Have you done the xbox live connection test from your xbox? If so what does the report say. I've read a few other people having their settings messed about after the recent live outage. e.g. NAT setting set to strict. If your xbox and the modem have worked previously, then something has introduced a change. Do you have to have you PC on to get your modem working or is it standalone?
Hope you can get it sorted quickly. It must be damn frustrating, especially with COD5 beta out.
Edit: A quick google search indicates that NTL customers are likely to have NTL200 (grey) or NTL250(blue). Both these are manufactured by Ambit. You can access the modem's config screens by typing in
http://192.168.100.1 into your fave browser.
This will bring back a log in screen prompting for User and Password. Both of these are 'root' (without quotes)
This should then get you logged in to a settings screen.
If you get this far and all is good post back.