I suddenly don't have an IP address and can't connect to the Internet!

A friend came over about a week ago and started playing with my MacBook pro. For most of the week I kept dropping the wifi signal. Then yesterday it wouldn't connect anymore. I tried restarting it several times, and renewing the DHCP lease, and removing all the remembered networks. Tried running the assistant and diagnostics, with no help. It says I don't have an IP address and that's why I can't connect. Any help?