So, you have your own WordPress site, and you are using some theme you got of the internet. You upgrade to Internet Explorer 9, and bam, all your page headings dissapear. I had this exact issue on this site, as the theme I use generates the post hears using Cufon. It’s not a WordPress issue, and this could happen with any site that uses Cufon.
Thankfully it’s as simple as upgrading to the latest version of the cufon-yui.js file, which you can download by clicking here or visiting the Cufon home page at http://cufon.shoqolate.com and clicking the ‘Download’ button. Then simply upload to your site and all will be fixed (of course you’ll take a backup of the existing file just in case though, won’t you?).