Winter is setting in in the northern hemisphere of Mars, so there is little sunlight for Phoenix to continue operations. It haven't been heard from since Nov. 2nd, so it may be dead in the dark winter.
Now they can focus on the analysis of the amazing data, including that from the first atomic force microscope used on another planet. Who knows what we will find from it.
It had lasted longer than the planned 90 days. 
They'll try contacting it when it warms up again there, but there is not much chance of a reply because the lander is going to be totally encased in CO2 ice in temperatures low enough to ruin the electronics.
Phoenix Lander At Mission's End | Universe Today
NASA's Phoenix Lander stops phoning home from Mars
Mars lander succumbs to winter - International Herald Tribune
Goodbye Phoenix.
Now they can focus on the analysis of the amazing data, including that from the first atomic force microscope used on another planet. Who knows what we will find from it.
It had lasted longer than the planned 90 days. They'll try contacting it when it warms up again there, but there is not much chance of a reply because the lander is going to be totally encased in CO2 ice in temperatures low enough to ruin the electronics.
Phoenix Lander At Mission's End | Universe Today
NASA's Phoenix Lander stops phoning home from Mars
Mars lander succumbs to winter - International Herald Tribune
Goodbye Phoenix.
Goodbye Phoenix. 
Looks like Old martian winter has arrived there.
