1. An old man who is unable to stand should pray while sitting. If he is able to stand for the places where one bows down, he should do so. An ill man may even pray while lying down on his side if he is able to concentrate properly. (If he is unable to concentrate because of his illness, he should only recite the Shema, and concentrate on the first verse.) (Shulchan Aruch and Mishne Berurah 94)
2. An ill person who is unable to pray with his mouth at all, should think the words of the prayer in his heart, and it is possible that he has fulfilled his obligation. It follows that even if he recovers before the final time for praying that prayer, he is not obligated to pray a second time. (ibid)
