There are two methods for this. Try method one first:

http://www.ieng.com/warp/public/474/pswdrec_6xx.html

If it's a no-go, then the following always works ;)

To recover the password you do the following steps:

1. Reboot the Cisco 675

2. Access the device through the serial Console (Speed: 34000, 8, N,1)

3. Issue the break command, <CTRL>-C

4. The Cisco 675 should be display a prompt =>

5. Issue the command: ES 6 (Erase Page? 6)

6. Issue the command: M0 (Turn of monitor mode.)

7. Issue the command: go 8. The modem should reboot, with exec and ena 
passwords removed. *NOTE: You will also loose your entire config.

Bill