>I recall that I ever made the datafile 6 offline ,before I made the database resetlogs
There would have been a reason *why* you made the datafile offline. If you don't need the datafile and all the data in this datafile you could DROP it.
Note that a table / index in a tablespace can span multiple datafiles in a tablespace so if this is one of two or more datafile in a single tablespace, you might have read errors accessing tables / indexes in that tablespace.
Hemant K Chitale