scott account locked in SQL Plus

You must be coming from the good old days of Oracle 8 :) It was finally acknowledged that a non-trivial amount of production DB instances were running with that account and it's default password still in it's initial setup form which is why Oracle has eventually patched that security hole.

To your specific question - here's a link (first hit in Google search, actually) that explain it.

Edit: Pasting the answer from the link into here for your convenience:

Here's how to lock or unlock Oracle database user accounts.

SQL> ALTER USER username ACCOUNT LOCK;

SQL> ALTER USER username ACCOUNT UNLOCK;

You can use this SQL command to change the password and unlock an account at the same time:

ALTER USERNAME  IDENTIFIED BY Password ACCOUNT UNLOCK 

Login in to your DB with user SYS

SQL*Plus: Release 11.2.0.1.0 Production on Wed Jul 25 15:13:25 2012

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Enter user-name: sys as sysdba
Enter password:

then issue

alter user scott account unlock;

Then you will be able to login as scott.

conn scott/tiger