sometimes we will occurs power outage or server reboot unexpectedly, it was an un-clean shutdown to oracle server.

then when we try to access oracle, we will get ORA-12518, TNS:listener could not hand off client connection

SOLUTION:

1.) re-start the listener.
2.) get in rman or sqlplus to manipulate recovery.

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$ rman target 'sys/sys@<your-db-name> as sysdba' nocatalog

3.) startup as normal

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RMAN> startup;

or start step by step to check if any issues there.

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RMAN> startup nomount;
RMAN> alter database mount;
RMAN> alter database open;