As long as it is not a 3100 series, the router has a serial connection. If you have physical access to the unit, you can use console access. You can then reboot the router while console connection is on. There is a brief prompt indicating you have 5 seconds to press 'Esc' to enter Bootstrap. Once you get into bootstrap you do the following:
1. type bypass passwords (space) Enter
2. boot (space) Enter
The router will boot and load the startup-config, but no user password or enable password is required to configure the router by console. Go into configure terminal and enter the username [desired username] password "[desired password]" + Enter
If it is a NetVanta 3100 Series router, you have to set your computer's IP to 10.10.10.(anything but 1), then cycle power on the router with your computer connected to one of the switch ports, As soon as the Status light start to blink, press the reset button in with a paper clip. That should put the router in bootstrap mode so you can telnet to the unit and do the same steps.
Here is another post on how to deal with this issue: [video] - Password Recovery in AOS
I hope this helps.
Thanks... I still don't know what caused the password to become corrupted other than the upgrade process... But that process is what I needed!
The problem was special characters in password without quotes...
Sent from my Verizon Wireless 4G LTE DROID