Snow molds are fungi seen in the cooler times of the year and grow at freezing or near freezing temperatures. Snow molds can damage turf grasses from late fall to spring and at snow melt or during cold, drizzly periods after snow has melted. It causes damage like roots, stems, and leaves to rot when temperatures range from 25 to 60 F (-3 to 15 C). When the grass surface dries out and the weather warms, snow mold fungi will usually stop, this can reappear in the same area year after year when conditions favor development.
Snow molds are favored by excessive shade, a thatch greater than 3/4 inch thick, or mulches of straw, leaves, synthetics,and other moisture-holding debris on the turf. Disease is most serious when air movement and soil drainage are poor and the grass stays wet for long periods, e.g., where snow is deposited in drifts or piles.
All turf grasses grown in the Midwest are susceptible to one or more snow mold fungi. They include Kentucky and annual blue grasses, fescues, bent grasses, rye grasses, bermuda grasses, and zoysia grasses with bent grasses often more severely damaged than coarser turf grasses.
There are two types of snow mold in the Midwest: gray or speckled snow mold, also known as Typhula blight or snow scald, and pink snow mold or Fusarium patch. The two types are found in the same geographical areas in the Midwest, including Illinois.
Snow mold appears on matted-down sections of the turf. Look closer and if it looks like a fuzzy whitish substance on top of a matted-down spot or section. It may have a pink cast around the edge it may be starting to develop pink snow mold. This can make severe damage if air circulation is poor, if the lawn was long going into dormancy, or has continued shade from trees and/or snow cover and stays damp in the spring. Pink snow mold is the more severe damaging snow mold.
You should lightly rake these areas to allow for air circulation, and to keep matted thatch from preventing grass growing through.
More on snow mold and other spring problems
http://extension.illinois.edu/lawntalk/other/winter_damage_to_home_lawns.cfm
https://en.wikipedia.org/wiki/Snow_mold