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The most common reason for an old cat to suddenly have very dilated pupils is a condition called 'retinal detachment'- the usual cause of this is high blood pressure, which in turn is usually either caused by a kidney or heart problems.
The other possibility is that she has had some sort of neural damage- such as a small brain haemorhage or stroke- that has affected her sight.
My advise would be to get her to your vets for a check up today- they will very quickly be able to tell if she does have retinal detachment, and although this is pretty irreversible, a lot of cats cope quite well with being blind, as long as the underlying illness is addressed, so i suspect your vet would want to check her out with some blood tests for her kidney and thyroid function (thyroid problems can also cause high blood pressure)
I do hope she's ok