One of the most common mistakes when using carriers (UPS, USPS, FedEX, Canada Post) is not setting product weights.   Product weights are essential for carriers to return rates, and they will return no rate when set to 0.  




Another possibility is that you have set up an invalid "Shipping from" address, or an address that the carrier does not recognize.  You can change this in your Preferences --> Shipping, and scroll beneath the various carriers until you fine the shipping from address.



Also see My Carrier Calculated rates seem too low / high. How can I edit my UPS / FedEx / USPS rates?