Restart the router and interrupt the Router loading process by using Break Command and boot into ROMMON Mode.
In ROMMON mode, change the default configuration register value to 0x2142 and reload the router.
Save the old configuration from startup-configuration to running configuration.
Change the enable Secret/password and save configuration from running to startup.
Reload the router and boot into ROMMON mode again and revert the default configuration register's value to default one i.e., 0x2102 and reload the router.
using following command
confreg 0x2102