Resolution:
The cause for this is mainly the Tomcat ports of the Admin domain being occupied by some other process running on the machine.
You can verify by looking into the tomcat.log located under &ltAdmin-home>/domain/&ltdomain-name>/tomcat/logs folder. You will see an exception like

ERROR main org.apache.catalina.core.StandardServer - StandardServer.await: create[8005]:
java.net.BindException: Address already in use
    at java.net.PlainSocketImpl.socketBind(Native Method)
    at java.net.PlainSocketImpl.bind(PlainSocketImpl.java:359)
    at java.net.ServerSocket.bind(ServerSocket.java:319)
    at java.net.ServerSocket.&ltinit>(ServerSocket.java:185)

Ask the customer to do netstat -a to look for the process which is occupying the Admin port. Kill the process and try restarting the Administrator, this should resolve the issue.