WordPress 3.5 has broken my ADD MEDIA button!

At the moment I can’t create blog posts as I uploaded the new version of WordPress and it has broken the image upload in my theme and I can’t find a fix! grrrr… anybody know of the fix for this without re-uploading the older version? its happened to quite a few people apparently.

I’m not a techy guy so if there isn’t a simple solution I’m in real trouble! I’ve de-activated the plugins and nothing changed, I have my own theme so cant use twenty eleven. For people who aren’t techy it seems WP and themes made for it is a regular minefield and nightmare to run. You get prompted to update stuff and it creates more problems than solves!!! Once this is fixed I won’t be updating anymore unless the old versions become obsolete!!!

Help!