Troubleshooting AJAX issues in WordPress is tricky. Use these two ways (and a tip) to debug AJAX issues while building your plugin or theme.
Do you know 100% if that 'simple' code change in your plugin broke something? Setup PHPUnit to add tests and get more confidence in your code changes.
Instead of altering an existing theme, the best practice is to create a child theme. This is the hidden secret to styling child themes for WooThemes.