The NoMachine client 4 can get the following message:

The connection to the server was lost.

Error: connect to host  xxx.yyy.zzz.www port 22: Connection refused

when trying to connect by using the SSH protocol but the SSH server is not available (Note that connections via SSH to server version 4 are supported only in the enterprise-oriented products: both the Enterprise Server and Terminal Server families).

This error message can be due to:
 

- SSHD is not installed on the server.

Be sure the SSH server is installed on the server machine. This applies to Linux and Mac hosts, the package for Windows installs the NoMachine SSHD server that can be stopped and started  from the NoMachine server settings GUI.

- SSHD is stopped.
  
Be sure that SSHD is up and running  on the server. if your server is installed on Linux or Mac, please use the sytem facilities for starting SSH. If your server is on Windows, you can start NoMachine SSHD from the NoMachine server settings GUI.

- SSHD is listening on a port different from that specified in the connection settings.

In this case, please edit your connection settings and specify a proper port for the SSH protocol.

It is also possible that the client receives the message: 

The connection with the server was lost.

when trying to connect using the NX protocol but the server has stopped.

In that case, be sure that the server is up and running, you can start it from the NoMachine monitor in your system tray: select 'Show the server  status' and click on 'Start the server'.

Note: Be sure that the filesystem where the user's .nx directory is located is not full and not corrupt.