Do you mean, force everybody to use a username/password if they want access,
or do you mean, is it possible to BREAK INto your website.
The answer is yes to the first, maybe to the second.
You need a secure server to keep others out.
Forced entrance is very difficult but not impossible. Security such as in online-banks is almost impenetrable, definitely for us normal earthlings.
But if you have a hacker's mind and enough computing power, you might find entry eventually.