Some possible causes for your dog's loss of vision are:
This is where the eyes are damaged by high blood sugar levels. Warning signs of diabetes in dogs are drinking excessive amounts of water, incontinence, lethargy and weight loss. WebMD's section on diabetes in dogs suggests that Miniature Schnauzers are prone to diabetes in old age. Your vet will be able to do a blood test to determine if your dog has it, and will help you with treatment. The vision damage is irreversible, but treatment is possible and can prevent further degeneration.
Progressive Retinal Atrophy (PRA)
This is a genetic disease that causes the light-detecting cells in the dog's eyes to die. Different breeds have different forms of the disease, broadly classified into early-onset (which affects puppies), and late-onset (which affects adults). However, late-onset PRA usually manifests much earlier than 11 years, so it's unlikely to be this. There is a genetic test for Miniature Schnauzers that could tell you for sure.
Glaucoma is caused by a build-up of pressure of the fluids in the eye, which eventually damages the retina and optic nerve. The damage is irreversible, but medication can slow or stop progression of the disease. Surgery may be necessary (either to relieve the pressure or remove the eye) if the disease has progressed sufficiently.
Cataracts are caused by damage to the surface of the eye over time. There are many causes, including injury, diabetes and infection. They can be visible as a cloudy patch in the eye and can be treated surgically.
If your vet doesn't have the specialized ophthalmology equipment needed to diagnose the above conditions, they should be able to refer to one that does.
Alternatively, if you have dog shows near where you live, you may find that they have a vet doing CERF (Canine Eye Registration Foundation) tests during the show: you may not be able to make an appointment if you're not actually showing your dog, but you should be able to make contact with them at the very least.