How do I reset the clean URLs?
When clean URL are enabled, the page at admin/config/search/clean-urls is like the following one:



When clean URLs are not enabled, you should see a description such as the following one, and a button to test the clean URLs ("Run the clean URL test"):


  Use URLs like example.com/user instead of example.com/?q=user. If you are directed to a Page not found (404) error after testing for clean URLs, see the online handbook.


Once you click on the submission button ("Run the clean URL test"), Drupal would check if the clean URL are possible to enable, and show you the checkbox to enable them, which should be not selected.
If that doesn't happen, the online handbook reported in the description has a link to How do I unset the clean URLs? where three methods are listed:


Run the following Drush commands:  


    drush vset clean_url 0 --yes



Run the following MySQL commands:  


    UPDATE variable SET value = 's:1:"0";' WHERE name = 'clean_url';
    TRUNCATE cache;



If previous update statement didn't solve it, some drupal version may need a different key : 


    UPDATE variable SET value = 'i:0;' WHERE name = 'clean_url';
    TRUNCATE cache;



Alternatively, you can modify the appropriate settings.php file to include the following line at the bottom (or similar code in the site settings array you'll see there).  

$conf['clean_url'] = 0;