ORA-12560: TNS:protocol adapter error | connecting as SYS user

Experts -

I'm getting the following error when connecting using windows SQLPLUS command line.

Connected to:
Oracle Database 11g Enterprise Edition Release 11.2.0.4.0 - 64bit Production
With the Partitioning, Oracle Label Security, OLAP, Data Mining,
Oracle Database Vault and Real Application Testing options

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D
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X

SQL> show user;
USER is "SYSTEM"
SQL> def
DEFINE _DATE           = "15-JUL-15" (CHAR)
DEFINE _CONNECT_IDENTIFIER = "ikbqas2" (CHAR)
DEFINE _USER           = "SYSTEM" (CHAR)
DEFINE _PRIVILEGE      = "" (CHAR)
DEFINE _SQLPLUS_RELEASE = "1002000100" (CHAR)
DEFINE _EDITOR         = "Notepad" (CHAR)
DEFINE _O_VERSION      = "Oracle Database 11g Enterprise Edition Release 11.2.0.4.0 - 64bit Producti
With the Partitioning, Oracle Label Security, OLAP, Data Mining,
Oracle Database Vault and Real Application Testing options" (CHAR)
DEFINE _O_RELEASE      = "1102000400" (CHAR)
SQL> conn sys / as sysdba
Enter password: *********
ERROR:
ORA-12560: TNS:protocol adapter error


Warning: You are no longer connected to ORACLE.

I can be able to connect as system but not as SYS.

Thanks,
Sve