Connected but no ip address on wired or wireless connection

I have a Linksys wireless router connected to a cable modem which was working fine last week.... Over the weekend I helped my mate sort out his wireless network (simple job didn't have to fiddle with settings) and now I have can't get on the internet back home (finds wireless network ok). So I must have done something to a setting somewhere...
I have since taken my macbook round to someone elses' house to check everything was ok and it connected to their wireless network and internet fine.

I have tried connecting straight to the router via a wire but cannot access the router configuration.

I can connect properly to the router by manually entering an address (connection colour changes from amber to green) but it says it is giving a self-assigned ip address so doesn't connect to internet (nor to the router config settings).

I have tried pinging the router address and my mac can't find it... (on wired)

I have tried all the usual turning off everything and restarting (and reinstalling router firmware). The router/internet is working fine with my housemates desktop so I figure it has to be something in my macbook.

I've exhausted my brain Any ideas very gratefully welcomed! Is there a way of completely resetting network settings or go back to factory configurations?

thanks
Mark