2 days ago, I got an error from a customer that the Joomla Administrator page login failure where you entered your login username and password, the page does not show any error but return to the login page itself.
Here is the solution I could find to fix this problem.
1. Log into the PHPMyAdmin if you got it.
2. Look for the table jos_plugins.
3. Search for User – Joomla.
4. Edit the entry.
5. Look for published and change its value from 0 to 1.