This is typically a result of a failed auto-update. Download WP3.4.1, unzip.
First upload a .maintenance file using an FTP tool such as Filezilla or your host provided site manager
Delete wp-admin and wp-includes folders (using an FTP tool such as Filezilla or your host provided site manager). ONLY upload now wp-admin and wp-includes folders. In the root of the download are several other required files, upload and overwrite these (as it does not include a wp-config.php or .htaccess files they will not be overwritten)
Now, upload a default theme (is in your download) to the wp-content/themes/ folder.
Rename any existing theme folders to old-theme-name
Rename any folders in wp-content/plugins/ to old-plugin-name
Run site. Site should now run.
To work with an issue with theme or plugins, first check their compatibility with your current version of WP or any known issues, then rename back first your theme, activate it in the admin dashboard, run site, then same with plugins, one at a time only until an issue occurs. IF one does, follow above to rename again.
