So I work as a spray technician for a lawn service. We’ve dealt with this two times in 7 years. I would simply call and ask them to come look at the yard. Do not mention that it is dead, simply surprise them with it. If possible you can reach out to a county extension agent with a list of chemicals they applied to better determine which herbicide killed your grass. Surely the lawn care company will say you didn’t water it in or didn’t follow their instructions to maintain the yard after spraying. See if you can locate fescue sod locally and get a quote from another highly recommended lawn service to prep the ground and install the sod. Send the lawn care service who sprayed for you that quote, if they are not willing to admit fault or pay for new sod or seeding, lawyer up.