Unless you have a female dog which has been using the lawn as a toilet, I would say a bad infection of chafer grubs could do that to it.

Or leatherjackets. Place a sheet of plastic on one of the patches and leave it overnight, have a look underneath in the morning, first thing - if its leatherjackets or chafer, you should see the grubs sitting on the soil surface.

Doesn't have to be a female dog, Owdboggy, the same happens when my male Springer goes on the lawn.

If the dead patches correspond with footsteps that somebody may have taken across the lawn, it could have been caused by walking on a snowy/frosty lawn, or an infection of fusarium patch, aka snow mould, which colonises cold wet patches of compacted lawn and then gets spread on footwear.

Fusarium what what I thought of too, especially if the patches started small and then spread. If its that it can be treated with a fungicide - ask your GC which one, I've forgotten.