This might help for some users having the inaccessible site issue:
Having the "maintenance mode" ON, depending on the plugin you're using, blocks external access, such as the one required by JetPack to connect through WordPress.com. Deactivating the maintenance plugin resolves the issue. Although, even if you deactivate the maintenance plugin you're using, connect Jetpack through WordPress.com, then reactivate the maintenance plugin, JetPack might still have a problem accessing your site/subsite.
This might be quite obvious for some, but it may not be so for novice users.