From Metalink:

Problem Description:
====================
When your server is not connected to the network and you try to connect in
Server Manager (svrmgrl), the connection fails returning an ORA-12570. If you
check the "sqlnet.log" you will find a TNS-12631 error.

ORA-12570 / TNS-12570: TNS:packet reader failure 
ORA-12631 / TNS-12631: Username retrieval failed 

Problem Explanation:
====================
These errors are associated with remote authentication on NT under NTS (NT Transport Services).

Essentially, the following parameter is set in the "sqlnet.ora":
SQLNET.AUTHENTICATION_SERVICES = (NTS)
As a result, if you are logged on as a domain user, and you don‘t have a network connection (network cable temporary removed or laptop booted standalone), you will not be able to connect to the database because the authentication will try to reach the domain users database on the PDC (Primary Domain Controller) or BDC (Backup Domain Controller).


Solution Description:
=====================
There a 2 solutions to connect to the database when no network is present:
1. You can log in as a NT local user. 
2. You can disable NTS in sqlnet.ora by setting the following parameter:
　　SQLNET.AUTHENTICATION_SERVICES = (NONE)

Explanation:
============
1. When you log in as a local user, then the local users database is contacted and not the remote domain users database.
2. When NTS is disabled, even though you are logged in as a domain user, the domain user‘s database will not be accessed.

ORA-12631 / TNS-12631: Username retrieval failed