I'm sure you checked the Area above the duct for water damage, IE the roof in the attic, I'm assuming It's the attic that the duct work goes through. It might be a simple Patch job If you can find the leak, however, with It being in the duct work, You could always go about 2' on either side of where the Snow built up, and replace that area.
Who know's it might be Frozen condenzation also, depending on the weather in your area, But I kind doubt that, since you'ld have been running the air during a freeze.
Stephen