We have an amazing situation that we don't have the lastdays backup (as backup was not properly taken up)
The database gets mounted but don't get opened as one of the data file is missing.
Is there any way to recover the datafile and make the oracle server live once again.

Good question! There are ways to bring the DB up.
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SVRMGR> alter database datafile '/u01/oradata/ORCL/data/users01.dbf' offline drop;
SVRMGR> alter database open;
SVRMGR> drop tablespace users including contents;
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In the first stmt you can either use DROP or just use OFFLINE.It totally depends on what kind of data is out in that datafile.If un-necessary data then just drop it...if u happen to have valuable data then you can bring it offline and try to recover the DB.
Also you haven't mentioned if you are running a COLD or HOT bkp ?
Do you at least have an EXPORT.dmp of the tablespace?
If you have then just export the Tablespace.
I am not sure this might workout for your scenario but it may bring the DB up for now.

Thanks a lot , by taking datafiles offline , server could be opened and is now live.
but all the files taken offline are showing files 0 kb size.
Was it due to virus or anyother reason?
and these files now require auto recovery.
can these files be repaired now
However thanks for the advice

I ran the following command and the output came like this
Can you explain me the reason for the following output and also how the files can be recovered.

SQL> set autorecovery on;
SQL> recover automatic datafile 11;
ORA-00283: recovery session canceled due to errors
ORA-01157: cannot identify/lock data file 11 - see DBWR trace file
ORA-01110: data file 11: 'E:\ORACLE\ORADATA\HHML\D_HH_ATEND.ORA'

Seems like the only option in your scenario would be dropping the tablespace and re-creating a new one.

If you happen to have an export dump for the tablespace or objects in that tspace just import them after creating a new ts.