It does look pretty sad. Start by cutting out all the dead branches. You can test whether a branch is dead by scratching the surface of the branch. If the layer underneath is green, its alive. If brown or hard, its dead. A branch with dead leaves could still be alive so test. Dead branches also snap off, a live branch will bend.
We also have to figure out why your tree is suddenly dying. Has it been moved? Watering changed? How much heat does that radiator put out?
What is that mass of brown at the base of the branches? Dead leaves? Even if everyone of those branches is dead, if the trunk is alive, the tree will live. So scratch test the trunk also.
A word of caution: Never, ever fertilize a plant that's struggling, that would be a death sentence. Fertilizer is for healthy plants.