Data File deleted using OS utility

If backups are available then

mount the database and
ALTER DATABASE CREATE DATAFILE old datafile name as new datafile name;
or
ALTER DATABASE CREATE DATAFILE datafile# as new;

recover datafile datfile name (or datafile#);
open the database

If backups are not available then remove the datafile entry so that database can be opened.

mount the database and

alter database datafile datafile# offline drop;
or
alter database datafile datafile name offline drop;

open the database and drop the tablespace if no longer needed.