Assuming there is no problems with the network connection between the client and the server, it is time to check whether the server is running properly.
A useful tool to determine whether a process is running is ps. Use its man page to learn all the amazing options. A particularly useful way to use ps is “ps -Af”. This command lists all the processes, along with the command line that starts each one. You can pipe it to less so that you can go back-and-forth:
Most server daemons are named after the protocols, but some are not. You need to know the exact name of the daemon for a particular protocol. For example, apache2 can be the name of the process for HTTP serving, slapd for LDAP, sshd for SSH and etc.