Before starting up the Denodo monitor you need to check the connection parameters to Virtual DataPort Server in the configuration file: <DENODO_MONITOR>/conf/ConfigurationParameters.properties
If you have recently modified the IP address/host name of the machine, port number and authentication details you need to reflect the same in the properties "vdp.jmx.host","vdp.jmx.port","vdp.jmx.user" and "vdp.jmx.password".
I have found more useful information in the [Denodo Monitor]( https://community.denodo.com/docs/html/browse/6.0/vdp/administration/monitoring_the_virtual_dataport_server/denodo_monitor/denodo_monitor) section of the documentation.
If you are a user with support access and you continue having problems dealing with the Denodo Monitor, I would recommend you to raise a support case in the Denodo Support Site (http://support.denodo.com) by attaching the complete log file. Your issue would be analyzed in detail by the Denodo Support team and they will help you to find the solution.
Hope this helps you!