Areas of my lawn that were covered with snow for most of the winter aren't greening up. What could be wrong?

FAQ
Question: 

Areas of my lawn that were covered with snow for most of the winter aren't greening up. What could be wrong?

Answer: 

Snow molds may be responsible for the brown spots in your lawn.  Two snow mold diseases, gray snow mold and pink snow mold, occur in Iowa.  Gray snow mold is caused by two species of the fungus Typhula, while pink snow mold is caused by the fungus Microdochium nivale

Symptoms of snow molds first appear when snow melts in late winter or early spring.  Circular, straw-colored patches appear in the lawn as the snow recedes.  Patches caused by gray snow mold may be a few inches to a few feet in diameter, while those caused by pink snow mold tend to be smaller, less than six inches across.  These patches may continue to enlarge if the grass remains cool and wet.  Grass in the patch may be matted and wet, with pink or gray colored fungal growth over the patch or on the edge.  Gray snow mold causes small, pinhead-sized, round structures (sclerotia) to develop on the leaves and crowns of the grass plants. 

Damage caused by snow molds is usually not serious.  Affected areas typically green up, though more slowly than the rest of the lawn.  Gently raking the affected areas may help to dry them out and prevent further fungal growth.  In future years, problems with snow molds can be minimized by avoiding excessive nitrogen fertilization in fall, keeping the grass mowed until growth stops in fall, raking fallen leaves, and by not throwing or piling snow on problem areas.  Affected areas will need to be overseeded if they fail to green up.