This happens very often (or something like it). Someone specifies a specific page on your DNN portal to be a login page, and then someone either deletes the login module, or deletes the login page. Once they logout, it appears that there is now way to get it back. This is when many people panic and turn to the DNN forums for help.
The truth is, fixing this is very easy. Really! It is.
First, navigate to ANY page on your site. Then, you can get your login just by editing the URL. For instance, let's take the typical home page URL. It normally looks something like this:
The URL parsing in DNN allows for a great many things to happen. One of them is the ability to change the content of the page to use or display a different module on the very same page. For instance, if you change the preceding URL to include another piece of text (ctl/login), your login module will magically appear on the page. The edited URL would look something like this:
That's it! Unless you have a problem that is more deeply rooted, this should get you to the point of being able to fix the problem.
UPDATED ON 07/09/2011
As of the version 5 series of DotNetNuke, you can now use another method to access the login page. As long as the website is using the Human Friendly URL setting, you can use the following example URL.