Now the 127.0.0.1:8000/admin/ site isn't available.
No estás haciendo la referencia al archivo por el nombre. Renombrado tu archivo HTML a base.html
This suggests the server is not running, can you get to http://127.0.0.1:8000/ ?
is your dev server (runserver) still running?
Did you make sure to uncomment the admin auto discover in urls.py and installed apps in settings? I find it easy to overlook those things and causes major headache.
ImportError: Could not import settings 'myproject.settings.admin' (Is it on sys.path? Is there an import error in the settings file?): No module named myproject.settings.admin