If you can ping an address
ping
www.bbc.co.uk
say and get a response then DNS name resolution is working ok. Just to be sure try something a little more unusual just in case the address has been cached. I tend to use
www.cisco.com on the grounds that unless you happen to work in IT it's very unlikely that the website has been recently viewed and the address has been cached.
If the ping responses fail then you have a DNS problem.
If this is ok then in the command prompt try the telnet command. If you get a successful connection then the internet connection is basically working.
Lastly try entering
www.bbc.co.uk into the web browser. If it fails then the problem is almost certainly a failed proxy. If you don't manually specify a proxy then your web traffic is routed via a transparent proxy which is supposed to be set up in such a way that NTL can load balence. Generally it works just fine but can sometime go a bit wrong. I recently had a real problem with Proxies in the south London area and had to work around them but it all seems to be fixed now.
There is a help guide somewhere on how to manually set a proxy for IE and Firefox etc. and what addresses to put in. I can't remember where it is but I'm sure someone else will have the info and post it. Don't forget to try taking the manual proxy setting off after a few days though.
Good Luck