If this is indeed weed-killer damage, those work by putting them on green plant which carry the poison down to the roots, with the intent that they don't grow back. Time will tell.

It could be an infestation of the roots by a bug of some sort. In that case, take a sample (with leaves, stems, roots in a sealed baggie) to your county extension service (find that service simply by typing "county extension service" with your county name into a search engine.

If it does not spread into the reset of the lawn, this could be an opportunity to do something interesting with that front portion of the yard, such as an undulating delineation with the same cobbles you have at the back of the lawn, plus gravel and desert plantings that will thrive in the reflected glare off the sidewalk. It could make your house feel a bit further back from the street, too. It would require removing the dead sod and laying down a weed screen, and adjusting the irrigation so you don't drown the new plantings.

Good luck figuring this out and coming up with an attractive solution that you'll be glad to come home to every day!

Sure looks like the yard people who sprayed for weeds used previously used weed killer. It will take a year to be able to successfully plant there. It’s a good opportunity to make a curved bed all along sidewalk as previously suggested. A mix of large bolders, drought resistant plantings and some attractive pebbles or crushed granite for this area.