连接到oracle时出错

Question

连接到oracle时出现以下错误：

C:\Users\nikhilgupta>sqlplus / as sysdba

SQL*Plus: Release 11.2.0.1.0 Production on Sun Jan 19 21:27:54 2014

Copyright (c) 1982, 2010, Oracle.  All rights reserved.

ERROR:
ORA-12560: TNS:protocol adapter error


Enter user-name: nikhil
Enter password:
ERROR:
ORA-12560: TNS:protocol adapter error

请帮助。