If you accidentally activate (or if it gives you trouble for any other reason) the "Force SSL" option under "System" -> "Global configuration" -> "Server Settings" (pictured below), you won't be able to login to your Joomla! administration page (or even the entire site).
In order to restore the default configuration (i.e. to set the above value to "None"), locate the configuration.php file in your Joomla! installation directory (with you web-based server administration panel or via FTP) and look for the following line:
public $force_ssl = '0'; (it should be reset to '0', as it is here)
Save the file and try reloading the administration login page - you should be able to login normally again.