Create an error!

Simulate a datafile was lost accidently.
shutdown database then rm test01.dbf.
startup the database again.
SQL> SQL> SQL> SQL> startup
ORACLE instance started.

Total System Global Area 1224736768 bytes
Fixed Size 1279916 bytes
Variable Size 536873044 bytes
Database Buffers 671088640 bytes
Redo Buffers 15495168 bytes
Database mounted.
Database opened.
SQL> conn test/test;
Connected.
SQL> select count(*) from test;
select count(*) from test
*
ERROR at line 1:
ORA-00376: file 6 cannot be read at this time
ORA-01110: data file 6: '/home/oracle/test01.dbf'
SQL> conn / as sysdba
Connected.
SQL> select * from v$recover_file;

FILE# ONLINE ONLINE_
---------- ------- -------
ERROR CHANGE#
----------------------------------------------------------------- ----------
TIME
---------
6 OFFLINE OFFLINE
FILE NOT FOUND