Hi m. hasan,
In order to handle the network disconnect at client side you could leverage the 
StatusChangeEvent.
By setting a custom handler it is possible to perform actions related to connection and disconnection occurrences.
Please you could also refer to our example 
"Lightstreamer - Stock-List Demos - Flex Clients" and in particular to the implementation of 
this method .
Alternatively, if your application runs in a browser, you could rely on navigator.onLine property.
Regards,
Giuseppe