Tried to update to new version. Website and admin went down, error message states "Undefined class constant 'Client'". How to solve this message?
This will be a bug in a plugin you've got installed on that site that is trying to reference the non-existent (as of a few hours ago) Craft Client edition.
If you look in your
craft/storage/logs folder for that error, then you'll be able to see which plugin is causing it in the stack trace leading up to the error.
You'll then need to contact the plugin developer and share the stack trace with them so they can get it fixed for their next release.