After a upgrade joomla ! administration web page comes out without the login prompt, however I can see the logout button when I try obtaining the sign in screen.
www.yourdomain.com/administrator.
To resolve this error Open helper.php file under:
/home/username/public_html/administrator/includes/
change line 30 : if (!$user->authorise(‘core.login.admin’)) {
to
if ($user->get(‘guest’)) {
Save and quit.
If you want to find out your Joomla version you can check How to Find Joomla version from backend.