All those new water heaters with the semi-sealed fire boxes have a problem. It's called GOVERNMENT do gooder restrictions. Prepare yourself for sticker shock and call A.O.Smith tech support to have them talk you through the relighting of your unit. They will need the model number and maybe the serial number that are on the data plate. Also clean around the heater very well; no paint cans, boxes, toys, tools, clothes, etc. and vacuum under and around the heater very well -no dust or dust bunnies under the unit, no wood or paint or drywall dust around the unit of any kind, almost as in sterile. Then call their tech support. There is an air shutter and a ceramic fixture on the bottom of the firebox that is sensitive to almost everything flammable that can trip and shut off all incoming air for the flame. Besides that if the spark ignitor, or thermocouple go bad the entire firebox assembly has to be replaced; not just the bad item AND on the ones we were installing back when this all started that firebox cost almost a third of the price for a new heater. It's supposed to be for our safety, but my first heater lasted 27 years and I only changed it because we needed a little more capacity. My second heater has been in for 11 years and is still working perfectly without any messing with it.