I've seen what's in the pictures a few times and it does generally come on suddenly and usually tenderfootedness is associated with it.
Take care of the thrush or whatever is causing the heel pain (an abscess could also be the culprit) and get her landing on her heel again and the toe wall will again grow down tight and the separation will grow out.
Please don't treat it with chemicals, or anything you wouldn't be willing to gargle with, including bleach! There are so many caustic thrush remedies on the market. I use "gentle" iodine.
You might also want to take a look at your horse's diet and see if any improvements can be made. Let me know how it goes.