Login error after upgrading WP

My site was working fine until I made the mistake of updating WordPress. The site (http://davesink.com) looks pretty much the same with the exception of the ‘Latest posts’ plugin not showing up above the archived posts on the homepage as it did before. Also there’s a huge gap between the title and post content.
Now I can’t log in to my site. Instead I get this error message:

Warning: Missing argument 2 for wpdb::prepare(), called in /home/53/32/3003253/web/wp-content/plugins/category-icons/category_icons.php on line 1047 and defined in /home/53/32/3003253/web/wp-includes/wp-db.php on line 1210

Warning: Cannot modify header information – headers already sent by (output started at /home/53/32/3003253/web/wp-includes/wp-db.php:1210) in /home/53/32/3003253/web/wp-includes/pluggable.php on line 1207

I backed up the database and content, but do I need to trash the WP install and go back to an earlier version? I’m not a coder, so if any help can be framed so it would be understandable to a second grader, it would be much appreciated. 🙂

Thanks for any and all help.