We've had a few support tickets opened today by people with WordPress reporting their site as 'down'. Their sites were down but not due to anything we did or could have prevented. In every case it was a bad JetPack [a WordPress plugin] update that caused an error. Removing and re-installing the plugin resolved the issue.
I believe you can simply remove the wp-content/plugins/jetpack [or something similar] folder and then re-install via the wp-admin but you will want to view the error itself prior to taking action. Understand this is a webmastering issue and you should contact your webmaster for assistance if you aren't sure how to handle this.
To display the error prior to making any changes: