It's hard to diagnose without opening it up, but I had something similar happen to a Ringle hub about 5 years ago while mountain biking. Basically, one of the pawls that usually ratchets, had become broken and jammed, so that I couldn't coast without the hub making a massive metal grinding noise.

I don't do hub or headset work, so my mechanic took hub body apart, replaced the broken pawl and I've been using the hub ever since (even though I don't really use those wheels anymore).

Looks like this guy has pictures of his broken hub.