Step 1: Connect to the Router
Connect via Console: Use a terminal emulator (like PuTTY) to connect to the router’s console port.

Step 2: Reboot the Router
Power Cycle the Router: Unplug the router, wait a few seconds, and plug it back in.

Step 3: Interrupt the Boot Sequence
Enter ROMMON Mode: As the router starts to boot, send a break signal (usually Ctrl + Break or Ctrl + C) right after the router begins its startup sequence. This will interrupt the boot process and bring you to the ROMMON mode.

rommon 1 > confreg 0x2142

We set the configuration-register to 0x2142 with the confreg command. This tells the router to ignore the startup-configuration when booting. The router is nice enough to tell us we should reboot, so let’s follow its advice:

rommon 2 > reset
The router will boot and load its IOS image. Once it’s ready, you will see the following wizard:

Because the router is not loading its startup-configuration it will show you the wizard that is shown when there is no configuration. Just type no, and you will end up at the command-line. We will now go to enable mode:

Router>enable

Last Step – Call Router Old Configuration File Commnd 

startup-config to running-config

enable secret cisco 123 <New Password>

write

Go To Router config-terminal Mode Commnd

config-register 0x2102

write