Listen to her...ask her stories about her life and really listen. If she is unable to speak, then you talk about your life; tell her stories...especially ones of you and her. AND read to her; not news articles, but stories and/or books. Nothing feels better than someone spending time with you:)
First of all, remind her that you love her and that her family cares for her. Next, tell her about your life and show her photos. I know grandparents love hearing about their grandchildren's lives! Also, entertain her by showing her things you do (example: play music for her if you're a musician, paint something for her, dance for her, etc.). Lastly, if you are Jewish or Christian, read her some inspirational verses from the bible. I hope your grandmother feels better!