Frost can badly damage a lawn for multiple reasons. Ice and snow accumulation can make the lawn more vulnerable to breaking, and it also can over-saturate the lawn's roots with moisture and drown them. Heavy snow piled on the lawn can also result in snow mould that kills the grass outright. Caring for your lawn after a frost can help it survive and thrive as the weather gets warmer.
Stay off the Lawn
Frost literally freezes the grass so it stays stiff and is not pliable when weight is applied to it. As a result, stepping on the lawn while it is still frozen can result in the grass blades snapping since they can't bend properly. Therefore, walking across the lawn when it is still affected by the frost can badly damage it. Stay off of the lawn until it is fully thawed. Once all the frost is fully melted, you can begin walking across the lawn to treat it.
As the lawn recovers naturally from the frost, new grass will start to grow underneath the dead or brown grass. Accelerate the process of getting your lawn green again by gently raking your lawn to pull the dead and brown grass out. That will make it easier for the new grass to grow in while also removing the browning that makes the lawn look less attractive and unhealthy. Wait for the lawn to dry before raking since doing it when the lawn is still soaked can disrupt the recovery.
Fertilise at the Right Time
Fertilising the grass too early after a frost can result in a badly damaged lawn because the grass will start growing before it is properly ready, and the growing lawn can refreeze if the winter hasn't finished yet. Wait to fertilise until you're sure the freezing weather is finished. Fertilising then will provide nutrients that can help accelerate the lawn's healing and increase the speed with which it can grow back.
Let it Grow
Don't mow the lawn too early into the spring because the mower could be too caustic on it at that point. Allow the lawn to grow once you fertilise it. If you cut the grass too close to the soil, especially when it is first recovering from the cold, the trauma could result in the lawn dying off since it's still not healthy enough to regenerate itself effectively from trauma. Once the weather is warmer and the lawn is green and growing comfortably, resume normal mowing.