When you change themes on a Drupal 7 site, you often need to reset the placement of your blocks. If you're using the core block module to place them, you may end up with a bunch of blocks in unexpected places on your new theme. If you want to reset everything and start from scratch, you could edit the {blocks} table in your database, but you can also do it with JavaScript. While viewing the blocks admin page (admin/structure/blocks), paste this line into your JavaScript console:

jQuery('#blocks').find('select').val(-1);

Read more