Hi Ashok,
Login to cloudera manager, and select the service tab.
In the services, Select the Action button on your right side and from the drop down select the Restart or Start option to start services of Cluster.
It will start all the services including the namenode.
Hope this helps.
Thanks.

I am using CDH3.Can you please provide the command to start the demons and what user id should i use to bring hadoop up.

I dont think we have cloudera manager in CDH3.

Hi Ashok,
We don't provide the support for Cloudera cdh-3.
But Cloudera CDH-3 does have Cloudera Manager.

To start and stop services in CDH-3, please check this link:

http://www.cloudera.com/documentation/archive/cdh/3-x/3u6/CDH3-Installation-Guide/cdh3ig_topic_9_3.html
Hope this helps.
Thanks