“Cannot connect to server”, but why?
First diagnosis is usually trying to ping the server, however sometimes it is useful to check if a particular port is reachable from your computer. So these are your tools:
  ping
gives you information if the server is up and on the time a packet needs to travel to the server and back
  nmap -v -A host
scans all open ports on the server and lists them. Useful to check which services are running.
  telnet host port if the port is reachable, the answer looks like this
    Trying 123.123.123.123...
    Connected to filesrv.nes.aau.at.
    Escape character is '^]'.
Instead of telnet, any other terminal program (for example putty) can be used as well.