Error while exporting data to flat file from database using sqlplus in UNIX
ORA-08103 object no longer exists

means you were querying a segment that has since been (re)moved.  The most common examples of this are:


Someone truncated the table 
Someone did 'alter table move' 
Someone did 'exchange partition'


all of which move a segments contents to a new place on disk, hence the data you were scanning "no longer exists"