I have 2 suggestions that haven't been mentioned yet. I agree that something happened while you were gone and your cat is still upset by it. Not using the litterbox has become a habit now too. It also could be incontinence or dementia issues too.
First, get some Feliway, the cat pheromone that helps ease stress. This will help ease the stress that your cat experienced when you were gone.
Second, I think you should take a very intensive approach to re-training her. Keep her shut in a small room with her litter box (like a bathroom) for a weekend. Of course, spend time with her, its not an exile or punishment. Give her almost no choice but to use the litter box. Then start letting her out, bringing her to the litter box after a nap, after playing, after she eats... just like you would a kitten. Praise her when she uses it, treats etc... And put her back in the small room when you won't be with her, like when you're gone during the day and at night. Keep the litter box sparkling clean too (you know how we don't like to use an unflushed toilet).
Its hard work, but if the alternatives are pee all around the house or not having your cat anymore, I think its worth it. Good luck!!