You can log on to oracle even if you do not know the password of a database user.

1. After sqlplus '/as sysdba'
SQL> select username, password from dba_users where username = 'enable ';
USERNAME PASSWORD
------------------------------------------------------------
ENCLE E5406F88BEEF8108

2. Change the password and log on
SQL> alter user encle identified by encle;
SQL> conn encle/encle

3. Start another session and change the password to the original one.
SQL> alter user encle identified by values 'e5406f88beef8108 ';
SQL> conn encle/iamwangnc

Tip: try to shorten the above process to reduce the impact time of the database.