Many dynamic sites have a login block in them have a login area somewhere, like that on the right.

Note that the location may vary.   It could be the home page, it could be a Members page, or it could be hidden in an obscure About subpage.

If your site has such a block you can browse to it and use it to login.

Most sites that where logins is only by a single site owner do not have a login area within the site and the site owner instead logs in through www.hotdoodle.com and uses the "edit" link next to their site on the My Stuff page.

Most multi-user sites have a login area somewhere, like that on the right.

If you do not have such a block you can use the emergency login page, and you can use this also help you recover your site from a fatal error. For example: You might delete all ways to login for the site; you might create a layout that is unreadable or unusable such dark text on a dark background; or you might delete the menu or large portions of the menu and are unable to navigate the site.