If the database is in noarchive log mode, online redo logs can (and will) be overwritten without making sure they are saved. This implies that a database cannot be recovered even if backups were made.
If the database is in log archive mode, the database makes sure that online redo logs are not overwritten before they have been archived.
archive log list;
select log_mode from v$database;
show parameter recover
Set Online redo logs, Archived redo logs, Flashback logs
alter system set db_recovery_file_dest_size=2G scope=both;
alter system set db_recovery_file_dest='D:\archivelog' scope=both;
shutdown immediate; startup mount alter database archivelog; alter database open; alter system archive log start;
LOG_ARCHIVE_DEST_1 = 'LOCATION=/disk1/arc'