This error is shown when SQL Monitor can't connect to the base monitor (also called the monitoring service). You might also see the error "No connection could be made because the target machine actively refused it".

The error has several possible causes.

The monitoring service is disconnected, or there's a problem with the network

Make sure the machine that hosts the monitoring service is turned on and connected to the network.

The monitoring service is stopped

To make sure the monitoring service is running:

1. Log into the machine that hosts the monitoring service. 
2. In the Start menu, type services.msc
   Services opens.
3. In the list, find SQL Monitor Base Monitor.
4. If the service is running correctly, its status should be Started.
   If the status is something else, or if there's no status, right-click the service and select Start.

The monitoring service connection details are incorrect

If the machine hosting the monitoring service or its port number have changed, reconfigure SQL Monitor with the correct connection.

1. Log into the machine where the web server is installed.
2. Open the SQL Monitor web interface.
3. Enter the connection details.

The web server isn't authorized to access this base monitor

See The web server isn't authorized to access this base monitor.