The listed commands assume you're logged into the system as the oracle owner user, this way you don't have to provide the password when connecting to the database.

The command below unlocks the account and sets a new password:

• alter user username identified by new_password;
• alter user username account unlock;

These commands can be combined as you can see below:

UNIX>sqlplus / as sysdba

SQL*Plus: Release 10.2.0.4.0 - Production on Wed Sep 29 12:59:37 2010
Copyright (c) 1982, 2007, Oracle.  All Rights Reserved.

Connected to:
Oracle Database 10g Release 10.2.0.4.0 - 64bit Production


SQL> alter user dbuser account unlock identified by <password>;

Step 2 Validate User connection

Now that the user has been reset, we can validate if a connection is possible

UNIX>sqlplus dbuser

SQL*Plus: Release 10.2.0.4.0 - Production on Wed Sep 29 12:59:37 2010
Copyright (c) 1982, 2007, Oracle.  All Rights Reserved.

Enter password: *********

Connected to:
Oracle Database 10g Release 10.2.0.4.0 - 64bit Production