Frost does not harm grass seed that is still dormant. If you have seeded your lawn and an early fall freeze threatens, the grass seeds will still grow as long as the seeds have not germinated.
If a frost occurs after your grass seed has germinated, but before it has matured, the seedlings will be damaged. Depending on the severity of the frost and the maturity of the seedlings, you may need to reseed after the threat of frost has passed.
A winter thaw provides an opportunity for reseeding. Apply the seed at approximately 20 percent higher than the recommended rates to account for some potential freeze damage. The freezing-and-thawing action of the soil helps to work the seed in and provides protection until the grass is ready to grow in the spring.