SQL instance stopped being monitored or it appears that no data is being reported; error:
"... failed silently : A transport-level error has occurred when receiving results from the server. (provider: TCP Provider, error: 0 - The specified network name is no longer available.) : The specified network name is no longer available "
This error should really have caused a connection failure alarm, but it doesn't. Instead shows some graphs not moving or not populated.
Enhancement request SOSSE-4639 has been submitted to Development for consideration in a future release Spotlight on SQL Server Enterprise to have the connection fail.