If the server's IP address changed, these are the first things I would look at:
The TNSNAMES.ORA file on the client -- does it have the IP address hardcoded? If so, change it. Does it use the machine name? If so, does the machine name resolve to the correct IP address on your client machine?
The LISTENER.ORA file on the server -- does it explicitly specify the old IP address as its listening address?
