database is connecting from apps tier in R12

Today i got the error below after running fnd_nodes cleanup.

ERROR:
ORA-01034: ORACLE not available
ORA-27101: shared memory realm does not exist
SVR4 Error: 2: No such file or directory

I was able to connect to database and autoconfig ran fine in db tier. But it was throwing the above error in apps tier and i was unable to connect to db from apps tier. database was up and running fine.
Honestly i was not sure about the cause of this error. As i checked the xml file. DBC file, tnsname and listener file i could not figure out anything.

Cause of the error :

This is due to db restricted session. In order to run adstats.sql i had to enable restricted session. But i dint disable it after adstats.sql. Thats what causing the issue.

Fix :

Simply i bounced the db once since i could not figure out the cause of the issue while troubleshooting. After that this error disappeared.

Also we can do the below to avoid this error and no need of bounce,

SQL> alter disable restricted session;