If you've lost a system page, don't fret. While these pages may seem gone forever, they're still intact-- we just need to tell your site where to find them.
In concrete5, system pages like Login, Members, etc are Single Pages. Each Single Page is unique, and serves one particular purpose (like logging into your site).
New Single Pages are created through the Dashboard. Go to Dashboard > Pages & Themes, then click the Single Pages tab. You'll see a list of all the current ones included in your Sitemap.
To restore your Login page, we'll need to create a new Single Page at the exact same path as the one you deleted. Scroll to the bottom of the list, and under Add Single Page, type the following:
Click add, and your page is restored at www.your-site-url.com/index.php/login. Pretty easy, right?
Note that for the Login page, we typed login in all lowercase, and we did not include a file extension (like .php). This is a page's "handle", and only one page is allowed to use it. As I mentioned above, this needs to exactly match the old page.
For example, to re-create a missing Members page, you'd type members. If you need to re-create a page with a space in the title, like My Profile, use an underscore: my_profile.