Lynne's suggestion is a good one.
Sometimes dogs just get used to one surface (snow, for example) and have trouble accepting a different surface (grass, in this case) as the proper place to eliminate. I've never seen it in an adult dog that has used grass in the past, but I've seen it a few times with puppies.
that Stella will respond to retraining quickly!
The only other thing I'd suggest is a vet visit just to make sure there's no underlying health issue. If a dog has a urinary tract infection, for instance, and it hurts to pee, she may start associating the pain with grass in the yard because that's where she hurts. So seeing a vet, or at least bringing in a urine sample for analysis, is a good idea.
Hope Stella is back to normal soon!