Follow these steps- 1.go to config.inc.php file and find - $cfg['Servers'][$i]['auth_type']
2.change the value of $cfg['Servers'][$i]['auth_type'] to 'cookie' or 'http'.
3.find $cfg['Servers'][$i]['AllowNoPassword'] and change it's value to true.
Now whenever you want to login, enter root as your username,skip the password and go ahead pressing the submit button..
Note- if you choose authentication type as cookie then whenever you will close the browser and reopen it ,again you have to login.