Login to you CPanel | On the home page scroll down to Databases and open phpMyAdmin
In the left pane of phpMyAdmin select your Joomla Database. From the same pane, select jos_plugins table
Now click on the pencil item to edit Authentication - Joomla row

In the 'Published' row change the value from 0 to 1
Make sure that Save is selected from the dropdown list and click the Go button.
That's it, you should be able to login to Joomla admin now