With the coming of winter your lawn went into dormancy or hibernation. During this time of dormancy, the lawn loses a lot of its mass. Over the winter your lawn sheds up to half of its root mass, grass blades, & runners as illustrated in this diagram from the University of Florida.
Once Spring arrives make sure not to panic when your lawn starts to come out of dormancy. You will start to realize its not as dense and lush as it was last year but give it time and that shall change.
When we head into early spring do not be anxious at the state of your lawn. Make sure to give it time to regain the root structure and top growth that it lost over the winter. Spring temperatures play a significant part in how fast the lawn will start recovering from what was lost in the winter. Even though your yard will begin to turn green, grass does not start growing very quickly until the temperatures get into the mid-80s. Some years this happens sooner then others.
Be patient. A few weeks of warm summery weather will stimulate the growth needed to make your lawn lush again!