Oracle passwords can expire and if your application uses any of those users it could suddenly stop working. You can check status of your user from the dba_users table using the following query
select username, account_status, expiry_date, profile from dba_users;
To solve this, you could:-
1.) Run the sql query alter profile default limit password_life_time unlimited; which will change the password_life_time to unlimited for users with default profile (you can check which profile your user belongs to from the dba_users table). Your password will never expire.
2.) If the above query doesn’t re-enable the user, you may need to manually reset passwords by logging into sqlplus.
After re-enabling the log-in, its a good idea to confirm the expiry_date and account_status by checking it out in the dba_users table.