The problem lay in that Mars is inclined much like the Earth in it's rotation, and, like the Earth, the north and south poles have whole periods of direct sunlight, followed by long periods of no sunlight at all. That is the Martian winter, which rapidly approached the Phoenix Mars Lander. The Phoenix Lander works off of solar power. Here is an image of the Phoenix Mars Lander with it's solar cell array fully open.
A severe dust storm also coated the Lander's solar cells with fine silica dust, making it that much harder to maintain the power supply. The mission managed to have 5 full months of operation, far longer than the original month time-table. Once the spring thaw arrives, the NASA scientists will try to restart the Lander, but they are not holding out much hope. The last signal was received on November 2nd. Here is the home-page for the Phoenix Mars mission.