JetPack (used on WordPress) released an update earlier today, causing Fatal Errors on all WordPress sites that have it installed. They have an entry in their support Forum here:
https://wordpress.org/support/topic/jetpack-fatal-error-15/page/2/#post-16154149
If you can log into the admin area follow this guide:
https://www.a2hosting.com/kb/installable-applications/optimization-and-configuration/wordpress2/wordpress-plugins/disabling-plugins-in-wordpress#Disabling-individual-plugins
You can easily log in to WordPress and disable the plugin.
If not you will need to:
They are working on a release to fix the issue. For right now your best bet is to disable JetPack by going into File Manager and then to your WordPress Directory. Under wp-content then plugins, rename the JetPack folder.
UPDATE: Issues with Jetpack have been resolved. If you continue to experience any issues with the service, please do not hesitate to contact our live support for assistance.