ORA-01135
Some user reported an error ORA-01135 when querying the database.
ORA-01135: file 1291 accessed for DML/query is offline
ORA-01110: data file 1291: '/u01/app/oracle/oradata/orcl/users01.dbf'
ORA-01135 means that the file to be accessed is offline, there's no way to perform query or data manipulation on it at this moment. The file could be a datafile or a tempfile.
Such error is sometimes inevitable, because DBA have to offline the file for doing some maintenance. For DBA, you should know the cost is necessary, all you can do is to get the file online as soon as possible.
Solution
As I said, user failures are necessary costs if you have decided to bring the file offline during business hours. If you do want a complete isolated environment for maintenance, you should enable restricted mode for your database or choose a right time to do it.