CRecon
This command reconnects pipes and structures of the selected network.
To use, call the command line CRECON.
Select pipenetwork.
Without this connection, it is not possible to interpret the network and therefore not possible to calculate the network.
This link appears if you try to manually reconnect the pipe:
Note: the yellow circle that appears next to the structure when approaching the end of the pipe structure.
Sometimes the Erase operation causes an internal error in Civil 3D, which makes unplug something.
If when trying 
CNUM and give much connection error, use this command.
It may still be necessary to make an AUDIT (AUDIT command autocad, say YES when it asks if you want to correct errors...) in your drawing, if this command will not recoup all connections.