ct9a is exactly right! If you are freaking out like I was after upgrading your Zen Cart and your vital FedEx shipping calc. is down...

Install the module as instructed in the download of the Fedex module.

After you do this, you will still have the old values from your previous version of Zen Cart in your Admin>Modules>Shipping>FedEx Ground/Express

To get it working again, REMOVE the two modules and then INSTALL them right after removing them. (Make sure you copy your Account # from the old info)

It got my modules back up and working with no errors so far.

Thanks for the tip!