Try to put the missing datafile offline. Try this :
I assume the database is shutdown.
Log as sysdba into sqlplus
(connected to an idle instance)
sqlplus> startup mount
database mounted
sqlplus> alter database datafile '/path/filename' offline;
sqlplus> alter database open;
Your database will then be opened, but the data in the datafile inaccessible.
If the datafile is part of a tablespace you don't care of, you can then drop the tablespace.
But you can't drop a datafile. To get rid of the datafile, you must drop the sassociated tablespace.
Hope this help.
