Fatal error in the output generally indicates either your theme doesn't like your version of WordPress, you have a plugin that doesn't like your version of WordPress, or you have an incomplete install/upgrade.
Have you tried:
-deactivating ALL plugins (yes, all) temporarily to narrow down the problem. If the problem goes away, re-activate them individually (one-by-one) to find the problematic plugin(s). If you can't get into your admin dashboard, try resetting the plugins folder by FTP or phpMyAdmin. Sometimes, an apparently inactive plugin can still cause problems (because the hooks remain unless plugins completely removed or some plugins stick around in cached files. So by renaming the folder, you break them and force them inactive).
-switching to the unedited default theme for a moment using the WP dashboard. If you don't have access to your admin area, you can switch to the default theme by renaming your current theme's folder in wp-content/themes via FTP or SFTP or whatever file management application your host provides. The idea is to force WordPress to fall back to the default theme to rule out any theme-specific issue.
-If the above troubleshooting steps do not resolve the issue, try manually re-uploading all files and folders EXCEPT the wp-config.php file and the /wp-content/ directory from a fresh download of WordPress. This should replace your core files without changing your content and settings in wp-config.php file and the /wp-content/ directory. Read the Manual Update directions first!