I suspect that you had an unique index on this table on an online
datafile and that this index has been dropped very recently. If this is
the case, bring the datafile online, recreate the index in a safe place
and you will be back to where you started from.
I am getting the following error selecting from the view
select count(*) from v_xxx;
ERROR at line 1: ORA-01135: file 5 accessed for DML/query is offline ORA-01110: data file 5: '/db011/oradata/.dbf'
It worked fine for are very long time even earlier today too.
The datafile is offline for many years already and we did not have problems before.