ORA-01092: ORACLE instance terminated. Disconnection forced.

[pando]

try this

shutdown the database
startup mount
alter database datafile 'MISSINGwhatever' offline drop;
select * from v$datafile;
check the status column, see if it´s offline
alter database open;
drop tablespace XXXXX including contents;

where XXXX the tablespace that owns that datafile.