Try telnet host 21
Does it say connection refused?
1) Is ur system i.e client in a different LAN than the one on which ur ftping. 2) Have the IPAddressing scheme, I mean the subnets changed.
3) On UNIX server the basic FTP security is set in /etc/ftpd/ftpusers or just /etc/ftpusers, in /etc/ftpd/ftphosts and in /etc/shells. The ftphosts file if doesnot contain ur systems' IP then it wont' allow.
4) Lastly, did u check on the UNIX server itself whether u can run the `telnet localhost 21` cmd and get a blank or connected screen. Just to check whether FTP server on it is responding.