If you don't have power at the end of your wire run, where it connects to the spa, then I'd suspect a bad connection between your man panel and the spa. That could be anywhere a wire is attached, including your tap breaker, the main panel, the spa panel, and your GFI. It's also possible that one of your breakers is faulty, or you have a bad wire, which is probably a long shot.
Let us know what it turns out to be.