macbook won't connect to wifi

I'm at my parents' house and my Macbook suddenly won't connect to the wifi. I have tried almost everything: reboot modem and router, reboot macbook, network diagnostics, manual settings etc. It looks like it connects to the router, but not to the internet! (Macbook works with ethernet cable and other PC and an iPhone work fine on the wifi.) In the network settings AirPort is yellow and says "self-assigned" IP address. What can I do to make this work? It worked fine before but not anymore...

I use Mac OS X 10.6.8