--do u have a valid backup of ur database and all archivelogs from the time u took the backup?
--If yes, check whether it is a hardware failure. If no, then go as follows:
create an RMAN session as follows:
c:\> rman target 'sys/password@service_name as sysdba'

if u r using recovery catalog with RMAN, then create RMAN as follows:
c:\> rman target 'sys/password@service_name as sysdba' catalog
'rman_user/password@service_name'

then;
RMAN> run
{
startup mount;
alter datafile offline;
restore datafile ;
recover datafile ;
alter database open;
}


if it is a hardware failure, then go as follows:

create an RMAN session as follows:
c:\> rman target 'sys/password@service_name as sysdba'

if u r using recovery catalog with RMAN, then create RMAN as follows:
c:\> rman target 'sys/password@service_name as sysdba' catalog
'rman_user/password@service_name'

then;

RMAN> run
{
startup mount;
alter datafile offline;
rename datafile to ;
restore datafile ;
switch datafile all;
recover datafile ;
alter database open;
}

once u have taken datafile offline, u can open the rest of database. The above one was just an example..