ORA-12541 TNS no listener error is generated by oracle network layer. This error occurs when the listener for the oracle server you are trying to reach is down or unreachable. This error cannot be fixed by the user and should be resolved by the DBA. You can use the document and the links below to perform the basic troubleshooting before you open a ticket with the database administration team

End User Troubleshooting for ORA-12541
If you are a end user trying to connecting to Oracle Database when you received an ORA-12541 error, you should validate the following items before opening a ticket for your database administration team. Please validate that you are actually getting an ORA-12541 and not a ORA-12154 error.

1. Is there a known maintenance outage that has been published and if yes you should wait till the outage ends.

2. Has your database been moved recently, if yes, did you update your TNS Names entry. If you are not sure about this, just ask one of your colleagues to see if things have changed since the last time you connected to this database.

3. Some times IP Changes in the server lead to delays in DNS propagation leading to this error. Validate if your colleagues are facing the same error.

DBA Troubleshooting for ORA-12541
If you are a database administrator and your users are facing ORA-12541 error, then use the following steps from the computer that has the issues.

1. Ask the user to ping the database hostname using the database ping utility. If the ping is successful, move the next step

2. On the database server, validate that the listener service is up and running. You can use the command "ps -ef | grep lsnr" to see what listeners are running on your server. you can also use lsnrctl status command to validate this.

3. On the database server, open your listener.ora file if you have one and validate the port number and the SID that is setup. If you are using auto registration, please validate that your service name shows up when you issue the "lsnrctl status" command

4. Ask the user to open TNSNAMES.ORA and validate that he or she is using the right port number. Its possible that the user has an invalid port number specified and its causing the ORA-12541 error.

5. If the user has incorrect setup of TNSNAMES.ORA, then it may actually be a ORA-12154 error. Please visit the ORA-12154 troubleshooting document.

Documentation for ORA-12541

ORA-12541: TNS:no listener
Cause: The connection request could not be completed because the listener is not running.

Action: Ensure that the supplied destination address matches one of the addresses used by the listener - compare the TNSNAMES.ORA entry with the appropriate LISTENER.ORA file (or TNSNAV.ORA if the connection is to go by way of an Interchange). Start the listener on the remote machine.