Seems to be okay in Firefox, but if it’s not working in IE, that’s likely because it is missing a DOCTYPE – that’s a MAJOR problem.
Looks like there are quite a few errors:
http://validator.w3.org/check?uri=http%3A%2F%2Fwww.chicagoscreenwriters.org%2F&charset=%28detect+automatically%29&doctype=Inline&group=0
This will get your started on fixing them – sounds like the header.php has some major problems – probably other files as well.
http://codex.wordpress.org/Validating_a_Website
You might want to consider switching to another theme that is supported as you may well run into more problems as WP evolves.
I had a feeling it wouldn’t be a quick fix. Any templates you recommend? And a tip for transferring everything over, or is it just a matter of starting fresh?
Yeah, that’s certainly an unfortunate situation 🙁 . All of your content will transfer to a new theme, but it will be some work to redo settings/options and other customizations. For sure, start off by making a Child Theme – so that your changes won’t be overwritten when the theme is updated:
http://codex.wordpress.org/Child_Themes
So far as a theme, I’d look for one that is current, has lots of users (see downloads number) and is well supported – check the forum for the theme. These are all good in that respect – in fact, all have their own dedicated forums and excellent free help from the developer:
Suffusion
Weaver II
Responsive
You’ll find those any many more great themes here:
http://wordpress.org/extend/themes/
The new default twentytwelve theme is excellent as well.