This comes to pass very frequently. When somebody indicates a detailed page on your DNN portal to be a login page, and then they either erases the login component or removes the login page.
Once they logout, it shows that there is current technique to acquire it reverse. This is when lots of people panic and spin to the DNN forums for help.
The fact is, fixing this is awfully straightforward. 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:

http://www.domain.com/Home/tabid/36/Default.aspx
The edited URL would look something like this:

http://www.domain.com/Home/tabid/36/ctl/Login/Default.aspx

As of the version 5 series of DotNetNuke, you can now use another method to access the login page.

http://www.domain.com/Login.aspx