Is there a statue of limitations when it comes to Medical Malpractice resulting in Death? What are the proper steps to beginning a lawsuit?

My mother passed away after she had simple surgery done at Dartmouth Hitchcock Medical Center in Hanover. The simple surgery was to correct a back issue that started when she was 5yrs old. My mother was taken care of the last 23yrs by the same doctor that did the surgery. When she was being operated on the doctor had her in a position that dislodged a blood clot and the following day she went into Cardiac Arrest for almost 30mins at Dartmouth before the nurses taking care of her found her. My mother had several medical issues such as diabetes, thrombosis (blood clots), and was functioning with half the lung capacity due to such lung damage from her being on a breathing tube when she had ketoacidosis years prior to this situation. My mother was on no monitors at the time of the incident even though she had all these issues. The doctor knew she had blood clots, yet no precautions were taken to prevent the dislodging of any. An autopsy was done and the outcome was "Natural causes".