How old is 'old'?? Indoor cats have a lifespan between 15-19 years of age.

If your cat is in the upper range of that you may want to have your vet check her out to be sure there isn't some sort of age related problem causing her not to use the litterbox. Anything from arthritis, to UTI, to dementia can be causing the problem which your vet will be able to help you with.

As for getting the odor out I personally use Vet Solutions Stain & Odor Eliminator, although many people here have had great success with Urine Off, and Natures Miracle.