At the command line; check if the listener is started and running correctly

lsnrctl status

Stop and restart the listener based on listener status

lsnrctl stop
lsnrctl start

Use the Enterprise Manager command to check the status of the Database Console

emctl status dbconsole

If you get an error “Environment variable ORACLE_UNQNAME not defined. Please set ORACLE_UNQNAME to database unique name”, please below command to set it accordingly

set ORACLE_UNQNAME=<dbname>
e.g:  set ORACLE_UNQNAME=orcl

If the dbconsole is already started and active, please restart the dbconsole with below commands

emctl stop dbconsole
emctl start dbconsole

Check the status of the console once again

emctl status dbconsole

You should now ideally be able to access the Enterprise Manager