Hello folks,
I've been successfully running a silverstripe website for a couple of months, until there was upgrade on the web server. Due to the change on .htaccess file, all the links on my site were modified and appear as http://www.mydomain.com/<subdir>/<page-title>;, that is, the subdir where included in the links. At this time, I was able to connect to the Silverstripe on http://www.mydomain.com/<subdir>/admin, but not on http://www.mydomain.com/admin. Now I've fixed the .htaccess file, but I still can't connect to the admin on http://www.mydomain.com/admin. I just return to the login page without any error message. I also found nothing about in the Apache logs.
How can I gain access to the Silverstripe admin?

In fact the problem occurred because I added the following code in mysite/_config.php.

Director::setBaseURL('http://my.domain.com/');

I solved the issue by replacing this code with

define('RELATIVE_SUBDIR', '<subdir>');

And I modified the method baseURL() in sapphire/core/control/Director.php

	static function baseURL() {
		if(self::$alternateBaseURL) return self::$alternateBaseURL;
		else {
			$base = dirname(dirname($_SERVER['SCRIPT_NAME']));
			if($base == '/' || $base == '/.' || $base == '\\') $baseURL = '/';
			else $baseURL = $base . '/';
			// Start custom code
			if(defined('RELATIVE_SUBDIR') && substr($baseURL, 0, strlen(RELATIVE_SUBDIR)) == RELATIVE_SUBDIR)
				$baseURL = substr($baseURL, strlen(RELATIVE_SUBDIR));
			// End custom code
			if(defined('BASE_SCRIPT_URL')) return $baseURL . BASE_SCRIPT_URL;
			else return $baseURL;
		}
	}

Now my url's are http://www.mydomain.com/<page-title>; and In can access the admin.
I work with SS v2.3.1.

I just installed a fresh copy of SS version 2.3.4. During the installation process I used 'admin' as the user name instead of an email address like it said I could. However, after the installation process, I followed the link to the CMS admin page and used the credentials it suggested. I receive 'I'm sorry, but you can't access that part of the CMS. If you want to log in as someone else, do so below'

wth?

I tried finding "Director::setBaseURL('http://my.domain.com/');" but it is not even in the file.

per IRC I added "Security::setDefaultAdmin("admin", "password");" in _config,php which allowed my to access the page.

Thanks guys for the quick response!