When you say you can't connect to the internet, do you mean wirelessly or wired or both? Is there a red light showing on the modem?
I have the same make/model of router. Sometimes with BT, it disconnects of its own accord. In which case, I log on to the router => http://192.168.1.1
- then input the logon & password (the default is Admin for both - but your friend may have changed this - in which case you will either have to get it from him [broken link removed]
Once you log-on to the router, you can check info ref. the connection. You may need to input the username and password (for the service as provided by BT) and click on connect.
Check that all out first - if its a wireless issue, post back here with more details.