At times I’m not able to restart the EM12c agent from the console. Fortunately command line works like a champ.
cd to the home directory of you agent bin directory
./emctl start agent
Oracle Enterprise Manager 12c Cloud Control 220.127.116.11.0
Copyright (c) 1996, 2011 Oracle Corporation. All rights reserved.
Starting agent ........................................ started.
ps -ef|grep emagent will provide the details of the process once started.
You can then reload the page within the console the agent should now display as up. If you don’t get an all clear with the green up arrow.
1. Click on the target of interest from the main target list page
2. Click on the word Agent with the downward triangle.
3. Displayed is a list of actions that can be performed on the agent. Select Resynchronization.
4. The Agent Resynchronization page will be displayed. Leave the check box by the unblock agent on successful completion checked.
5. Move over to the far right hand side of the page and click Continue.
Once this task is completed the agent should display as up.