Access to the front end of your Joomla CMS website can be restricted several ways. The obvious one is to not have the login module available. Obviously you can also turn off registration of new accounts in the backend as well.
One other thing to note is that the Akeeba Admin Tools extension has a security setting to enable or disable frontend access to the site for administrators. This is the most common reason that you are unable to login from the frontend.
Please go to the backend and check your security setting in Admin Tools.