Some time we come across the issue when trying to connect with database from the application side using apps as a user. This basically requires the listener to be up to make connection with database. We get error similar to
Cannot login to sqlplus using following userid/passwd.

sqlplus apps/apps@dbname.

Gives error
ERROR:
ORA-12547: TNS:lost contact
———————————————————
Error Messages:
ERROR:
ORA-12547: TNS:lost contact
———————————————————

Solution:
In this case first check if the database listener is up or now. If the listener is up then check the sqlnet.ora file present in ORACLE_HOME/network/admin also can be reached using the environment variable TNS_ADMIN. This file contains 2 parameters, which restricts login to the database from the other hosts.

tcp.validnode_checking = yes

tcp.invited_nodes=(hostname1, hostname2, … )

This will allow entry from hostname1, hostname2 servers only. So if these values are commented and listener is bounced then this will resolve the issue.