Make sure the account is not locked or expired.
SQL> select username, account_status, lock_date, expiry_date from dba_users where username = ‘TESTUSER1’;
USERNAME ACCOUNT_STATUS
TESTUSER1 LOCKED…
Reset and unlock
SQL> alter user TESTUSER1 account unlock;
User altered.
SQL> alter user TESTUSER1 identified by ‘MyNewPw#0721’;
User altered.
And here is a link to something that will generate sql to unlock and change bulk passwords, including reset to original – not tested
https://www.ateam-oracle.com/avoiding-and-resetting-expired-passwords-in-oracle-databases