Where do you live and what kind of grass mix do you have? From the trees I'd guess you either live at a high elevation in the mountains or in the north, but getting your nearest city or town would be helpful.

If you lived in the south and had bermuda, this happens a lot for lawns coming out of a cold winter. If you live in the north I'd say it was the result of snow mold. If you live in the far north, you could get away with reseeding even though we're approaching summer.

I'm in Boulder, CO. Will try the reseed! Thank you.