If you are using any of the Enterprise and Terminal Server product families and you receive the following message:

The NoMachine service is not available or the access was disabled on host ---.---.---.---

it is most likely because you are connecting using the SSH protocol and 'NoMachine login' when the NoMachine server has been stopped or shut down. The above message can appear in the following situations:

1) The server has been stopped, i.e. it doesn't accept new connections.

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'.

2) The server has been shutdown, i.e. all NoMachine services, included the Monitor in the system tray, have been stopped.

Be sure that all NoMachine services are up and running, you can start them from the NoMachine GUI: click on the 'Show the server status' button to open the Server Status panel and click on 'Run the server'.

3) SSH connections are disabled in the server configuration.

Be sure that SSH is listed in the ClientConnectionMethods key in the server.cfg file (e.g. ClientConnectionMethods NX,SSH,HTTP). Restart the server, even from command line or by Server Status GUI by clicking on 'Stop the server' and then 'Start the server'.
 

Note: if you are connecting to NoMachine 4 (free), please see this article:

https://www.nomachine.com/AR11K00751