Oracle Enterprise Manager 11g Database Control – Agent Unreachable

Single instance DB. Can login to OEM. However, it says Status – Agent Unreachable on the Home page.

Most tasks such as checking RMAN status, adding datafiles etc can still be done through the OEM. I do not know how "Agent Unreachable" impacts the OEM in this case (DB Control)

I checked status of the agent from $ ORACLE_HOME/bin/emctl status agent, it showed "The Agent is not Running"

So, I want to start the agent, but $ ORACLE_HOME/bin/emctl start agent is not working. The command is not available. In Oracle docs, it is mentioned that the command emctl start agent should be executed from AGENT_HOME/bin. However, I do not know the AGENT_HOME path.

Please help. Thanks in advance.