If your site does not have a login form on the front end for security reasons or they don't want to have one for other reasons, there is a way to get to a login form for the Front End of any Joomla site.

Add one of the following to the end of the URL for your site in the address bar of the browser.

/index.php?option=com_users&view=login
or
/component/users/?view=login

For Example:

http://www.mygreatwebsite.com/index.php?option=com_users&view=login

Keep in mind that all Joomla sites have this ability. You are not stopping people, including hackers, from being able to login to the front end of your site by un-publishing the login module.

If this is a common problem with your site, but you don't really want the login module displayed on your website, you can add a login page as a separate Menu Item outside your regular navigation menu.

You may have other contributors and article editors that regularly need to get to the front end, but again, you don't want the login module to appear on the regular site.

See this joom3 article: How to create a separate login page for my Joomla website.