What Happens If You Lost a Data File? - Oracle DBA FAQ - Managing Oracle Tablespaces and Data Files

After you shuting down an Oracle database, you accidently deleted a data file from the operating system. If you try to start the database again you will get error when Oracle tries to open the database after mounting the database. The following tutorial examples shows you what will happen if the data file c:\temp\my_space.dbf is deleted. Oracle can still start the database instance and mount the database. But it will fail on openning the database as shown below in a SQL*Plus session:

>sqlplus /nolog

SQL> connect SYSTEM/fyicenter AS SYSDBA

SQL> STARTUP
ORACLE instance started.

Total System Global Area  100663296 bytes
Fixed Size                  1285956 bytes
Variable Size              58720444 bytes
Database Buffers           37748736 bytes
Redo Buffers                2908160 bytes
Database mounted.
ORA-01157: cannot identify/lock data file 5 - see DBWR
  trace file
ORA-01110: data file 5: 'C:\TEMP\MY_SPACE.DBF'

SQL> SHUTDOWN;
ORA-01109: database not open
Database dismounted.
ORACLE instance shut down.