Check your WooCommerce System Status to make sure there are no red entries, especially in the Pages section
Test your checkout process with the console active
Check your server error logs for any entries that relate to the pay page
How to use the Google Chrome console :
https://developers.google.com/chrome-developer-tools/docs/console
How to use FireBug for FireFox :
https://addons.mozilla.org/en-us/firefox/addon/firebug/
Andrew, WooNinja
I work for WooThemes