I am always very annoyed when my Oracle user is automatically expired and locked. This is done automatically at 180 days in some installations.
The following are some very easy steps for a question that everybody is asking again and again on forums.
STEP 1: Connect to the DB
sqlplus / as sysdba
STEP 2: Update the password
ALTER USER myUser IDENTIFIED BY newPassword;
STEP 3: Unlock user
ALTER USER myUser ACCOUNT UNLOCK;