I haven't upgraded to the lastest version.
Sadly I think I read they removed the compatibility check, which I think is a bad idea.
Very useful feature because it would at least report a problem.
You may want to check if you're theme has the following :
<?php wp_head(); ?>
<?php wp_footer() ?>;
You can check by clicking
Appearance-> Editor in your admin bar.
Click Header and then after the Footer on the right hand side of your screen.
<?php wp_head(); ?> right before </head>
<?php wp_footer(); ?> right before </body>
Add them if they are missing and click Update file after you edit each before moving on the the next.