Plug in your serial console,

reboot the ASA
press 'ESC' when its booting

# Now get into configuration management
rommon #0> confreg

# Record this next line we'll need it later.
Current Configuration Register: 0x00000001
Configuration Summary:
boot default p_w_picpath from Flash

Do you wish to change this configuration? y/n [n]: Y
enable boot to ROMMON prompt? y/n [n]:n
enable TFTP netboot? y/n [n]:n
enable Flash boot? y/n [n]:n
select specific Flash p_w_picpath index? y/n [n]:n
disable system configuration? y/n [n]: Y

go to ROMMON prompt if netboot fails? y/n [n]:n
enable passing NVRAM file specs in auto-boot mode? y/n [n]:n
disable display of BREAK or ESC key prompt during auto-boot? y/n [n]:n

Current Configuration Register: 0x00000040
Configuration Summary:
boot ROMMON
ignore system configuration

 

Update Config Register (0x40) in NVRAM...
#now lets load the clean settings
rommon #2> boot


hostname# copy startup-config running-config
hostname# configure terminal
hostname(config)# password password
hostname(config)# enable password cisco123

hostname(config)# username admin password cisco123

# Remember that line I asked you to record 
hostname(config)# config-register 0x00000001
hostname(config)# copy running-config startup-config

 


# Reset to factory settings
config term
config factory-default

 

 

# Hit spacebar when the ‘more’ thing happens.
(Optional) reload save-config noconfirm