if you have lost one datafile. Then follow the steps shown below.
STEP 1. Shutdown the Database if it is running.
STEP 2. Restore the datafile from most recent backup.
STEP 3. Then Start sqlplus and connect as SYSDBA.
sqlplus / as sysdba SQL> Startup mount; SQL> Set autorecovery on; SQL> alter database recover;
If all archive log files are available then recovery should go on smoothly. After you get the "Media Recovery Completely" statement. Go on to next step.
STEP 4. Now open the database
SQL>alter database open;