Worst case scenario is root rot. This can occur with trees that are planted in dense, wet soil. Once severe root rot sets in, your tree probably won't be salvageable. Our soils were abnormally soggy this winter/spring. 
They can be susceptible to certain insects, including aphids and adelgids. If these insects are present, the tree’s needles might turn yellow or drop off. Forcefully spraying the needles with water to dislodge the insects every other day or so is a natural way to get rid of them. These are sucking insects and leave a sticky residue behind.
The sudden change from warm to really icy can also cause damage, especially if associated with wind. This would, however, generally affect exposed areas and not just the top.