O.K This could occour in two scenarios.

1. Scenario : You have set the archive log as true but haven't set the automatic archival. as a result when the redo log gets filled, the database awaits for the archival to take place. On this case you can check whether the automatic archival is enabled by

SQL> archive log list
SQL> select * from v$instance; check the archive column

Or do a manual archive;

ALTER SYSTEM ARCHIVE LOG ALL;

2. The case that the location where this redo is being written running out of disk space and as a result it couldn't write. OR your database is performing a high dml operations as a result your archive is getting filled pretty quickly.

So the simple solution for both the case is add more redo log groups or add more redo logs to each member of the group. Enable the automatic archival process.

Good luck,
Sam