Blocked Connection in listener

Blocked connection occurred when you listener is not registered with database, or when a database is in restricted mode and shutdown of the database is in progress.
If a database is in restricted mode, then LREG Background process block all connections to the instance.
Clients are getting one of the following errors:

ORA-12526: TNS:listener: all appropriate instances are in restricted mode
ORA-12527: TNS:listener: all appropriate instances are in restricted mode or blocking new connections
ORA-12528: TNS:listener: all appropriate instances are blocking new connections

The ORA-12528 error occurs when a database instance is not yet registered with the listener.

Solution 1
1. If the database service registration is not listed, then enter the following SQL command:

--1. Check the service is registered with listener if not follow the next step.

lsnrctl services

–2. Connect with the database
sqlplus sys as sysdba

–3. Fire following command to register the Listener with Database
SQL> ALTER SYSTEM REGISTER;

You can set the listener parameter for register the Server:

-- Mention listener name

ALTER SYSTEM SET LOCAL_LISTENER=listener_sales1;

–In RAC you can also set remote listener
ALTER SYSTEM SET REMOTE_LISTENER=listener_sales2;

cmctl: is used for register the listener on linux or unix system.

CMCTL> ADMINISTER [instance_name]

Solution 2
Check database is not present in mount, nomount & restricted state if not then start the database in open state:

Startup