I have Duplicate IP Addresses…”IP ADDRESS CONFLICT” on my home network

 "I desperately need that answer to just ONE question. A friend came over last night to try to get the DSL connected to the internet and somehow fucked up my router. The error message I receive is that there is an IP Address Conflict. What does that mean and how do I change it??!! I'm seriously stressed out here. I can connect thru the modem, obviously, and the other computer says the signal strength from the router is ""excellent"" so it's transmitting but it won't connect. AAAAARRRRRGGGGGGGHHHH!!!!! "