Hi Debbie,
Have you made any recent changes to your site that would correspond with this issue arising?
Is this issue arising for customers trying to ship to a country that you're properly set up to ship to in your Magento admin? (For instance, if you're only set up for the USA, and they're trying to ship to Canada, it's possible that there are no options available to them.)
Have you been able to replicate the issue yourself? Is it happening when you try to place an order through the Magento Admin panel, as well as on the front of the site?
Your UPS Settings in your Magento Admin Panel can be found at System --> Configuration --> Sales -- Shipping Methods, under the UPS tab. I'd recommend reviewing these settings.
Among these settings, there's a field called UPS Type. If this is set to "United Parcel Service XML", then fields should be showing with a specific UPS Access License Number, User ID, and Password. These correspond to a special account that UPS grants eCommerce websites for getting live rates based on your negotiated UPS discounts. With this method, it's possible that there's an issue with your specific UPS account that you'll need to call UPS to resolve.
In general, if you see anything that doesn't look right to you in these UPS settings, it may also help you flag the issue.
There's also a setting for Debug, which can be set to "YES". This enables error logging. In essence, a file in your website hosting account will be created and populated with information each time someone tries to get a UPS shipping quote via the shopping cart or checkout. This may give you a "smoking gun" as toward what's going wrong. It could be an issue with connecting to UPS. It could be an issue with your Magento account. It could be an issue of the addresses, weights, or some other parameter not jiving with what UPS is set up to offer your shoppers, etc.
Best of luck!
