Are the people on the server using a different TNSNAMES.ORA than you? Maybe that file is being maintained by your Oracle DBA, and s/he changed that file to reflect a TCP/IP address change or port change to your server that you are not aware of.
Make sure the connect parameters in your TNSNAMES.ORA file are current with the configuration of the Oracle server.
Mark McCasland
Midlothian, TX USA