ORA-12560 Oracle error indicates a high level error that may have happened in the oracle sqlnet or net8 transport layer. The actual error should be identified based on the other messages that oracle has displayed along with the ORA-12560 error message

ORA-12560: TNS:protocol adapter error

Cause:  A generic protocol adapter error occurred.
Action: Check addresses used for proper protocol specification. Before reporting this error, look at the error stack and check for lower  level transport errors.  For further details, turn on tracing and re execute the operation. Turn off tracing when the operation  s complete.

Some known issues with this error are

1. ORA-12560  and ORA-12500 during multiple connections to database : Most probably caused by a lack of resources on the unix server. ORA-12500 is thrown when a TNS Listener fails to start a dedicated process

Oracle metalink has a very detailed note on this error .. You can access that note in metalink by searching for note : 119008.1