Sometimes people have an epileptic fit, heart attack or other serious health issue in the air. This is part of life and passengers have been known to die in flight. The flight attendants are trained to deal with all these situations and they will be in communication with ground staff for extra medical assistance. Most airlines have defribulators on long distance flights. The purser may ask if there is a doctor or nurse on board to assist. If a passenger near you is such a state of distress, your job is to press the flight attendant call button to alert the flight attendants to the situation and then to be the least intrusive as you can be. Ordering a drink while flight attendants are trying to deal with an emergency is not very appropriate. In severe cases, the captain may take the decision to land the airplane at the nearest airport and the ill passenger (and any other passengers travelling with this passenger) will be deplaned first. Again be as disruptive as you can. It is not a good idea to be trying to get your bags from the overhead locker if an ambulance needs to come past you to attend to an ill passenger).
In case of a death, usually blankets are placed over the dead passenger and the airplane continues on or lands at the nearest airport.