My mother passed away Jan 2016. I am the personal rep. for her estate. She was 75 yrs old when she passed. I have not filed a tax return for her. What do I do now.

My mother passed away Jan 2016. I am the personal rep. for her estate. She was 75 yrs old when she passed. Her only income at the time of her death was SSI. I don't believe that she filed taxes for the last few years of her life. I also have not filed a tax return for her. What do I do now.