Visual Administrator "Error while connecting"

Hi people,

I have the next error when I try to connect to visual administrator, before it works fine.

Error while connecting

com.sap.engine.services.jndi.persistent.exceptions.NamingException: Exception while trying to get InitialContext. [Root exception is com.sap.engine.interfaces.cross.DestinationException: cannot establish connection with any of the available instances

Nested exceptions are:

com.sap.engine.services.rmi_p4.P4IOException: Cannot open connection on host: 10.0.0.20 and port: 50100

What can i do?

More thanks