We experienced the same issue today (somewhat sporadically; some extracts worked, some didn't). There was a Salesforce incident for at least part of the day. After deleting and republishing the impacted workbooks they worked. Not sure if it was timing or deleting + publishing, but they are working now for us.
Can you ask your Admin to open the workbook using Tableau Desktop on the Tableau Server machine and see if he is able to connect to Salesforce?
I would make sure to follow this:
I would also make sure your Tableau Server has not been firewalled off from reaching the Salesforce servers. I've seen that happen before where a whitelist needed to be created on certain firewalls. Some customers place Tableau Server in a much more secure place on the network than the end user desktops (who have full unfiltered access to the internet). You may have a network issue that is blocking access. Would need to check.