Resolving Magento 404 errors after theme changes can be a frustrating experience. These can usually be resolved by flushing the cache or reindexing, but after adding some en_GB language files the problem persisted. The Magento Community forums pointed to .htaccess issues, the need to resave ‘Roles’ and broken .phtml theme files, but for me it was a permalink issue.
Magento 404 errors
The need to resave your permalink settings is common in WordPress after adding new post types and the like, but this is the first time that theme changes have thrown my URL structure out. To resave your Magento permalink set up, go to the below in admin;
Unsecure >Base URL
> System > Configuration > Web > Unsecure >Base URL
Refreshing these resolved the 404 issue and brought my Magento site back online.
