Hopefully you have a Cisco console cable and com port on your PC (+fysical access to the controller)?
Put it in the console port on the controller and use following credentials user: password
password: forgetme!
Go into enable mode by typing "enable"
Enable password = enable
Type "configure terminal"
Reset the admin password
"mgmt-user admin root"
> choose a new password
Log out via exit
Now log in via admin/ new password
enable
show ip interface brief
Now you should have the ip adress and should be able to access the GUI with your credentials.