I have a client with a single furnace dual thermostats single level house. Both thermostats seem to be working but suddenly only half the house is getting warm air

I'm assuming there is a zone control panel and 2 zone valves. Check to see if the zone control panel is blinking an error code and also check that the zone valve is operating. Simplest way to check the zone valves is to reverse the wires from the zones on the control panel and set the t'stats. If the unheating zone now works, you probably have a bad zone valve.