As the lawn peeks through the receding snow winter-kill may be present in low areas, shady areas or spots that didn't fare as well as others for reasons unexplained.
Excessive snow cover or ice cover is usually the reason for winter kill, identified by a faded, bleached look to the grass. It can also occur on grass that was not snow or ice covered but exposed to the harsh winter wind. The grass can dry out and become dessicated.
Cool season grasses are hardy and resilient though and most times the grass is not completely dead, it will just take a little longer to recover than usual. The best thing to do is to rake the area, overseed and topdress with compost. The grass stitcher is also ideal for areas with winterkill.