ORA904 invalid identifier error all of sudden

We have a strange issue -

we have a user_authentication package in which below code is a part:
****
BEGIN
       SELECT force_chang_pwd_yn
       INTO lv_force_change_pwd_yn
       FROM ums_mt_user
       WHERE network_id = pi_network_id
       AND login_id = pi_loginid
       AND encrypted_password_text = fn_encrypt_user_pwd (pi_network_id, pi_password);
 
       IF lv_force_change_pwd_yn = 'Y'
       THEN
             po_error_code := 1;
       END IF;
 END;
*****

The below part of the code snippet started failing all of sudden from 0hrs GMT on 19th May 2013 with ORA 904 invalid identifier error.

SELECT force_chang_pwd_yn
       INTO lv_force_change_pwd_yn
       FROM ums_mt_user
       WHERE network_id = pi_network_id
       AND login_id = pi_loginid
       AND encrypted_password_text = fn_encrypt_user_pwd (pi_network_id, pi_password);

we are using oracle 10g standard edition - no upgrade happened in recent past on either our oracle server or our schema.

Strange thing is above SQL works fine if we run it autonomously. However, if we run the package it is resulting ORA 904 error.

Greatly appreciate a quick feedback on this.

Thanks in advance.