The specific error message is as follows: the database cannot be shut down properly, indicating a missing database file:
SQL> shutdown immediate;
ORA-01116: error in opening database file 6
ORA-01110: data file 6: '/data/oracle/product/11.2.0/db_1oradatatttdata.dbf'
The cause may be that the database file is corrupted or accidentally deleted, requiring a rebuild.
Solution:
alter database datafile '/data/oracle/product/11.2.0/db_1oradatatttdata.dbf' offline drop;
Drop the file, and then restart the database again.