As well as a bit of background to the problem (applications involved, environments etc), you should provide at least the following:
- Has it ever worked?
- Have you changed anything recently?
- Source IP
- Destination IP
- Protocol (TCP or UDP etc) if it isn't obvious from the context
- Port if it isn't obvious from the context
Common causes are:
- Wrong information - are you connecting to the right host?
- Network or system outages
Have you tried the following?
- Doing a host, dig or nslookup if dealing with a domain
- Pinging the remote host
- Connecting using netcat or telnet
It is probably worth providing the output to the above commands if possible.