Two quick things to rule out before calling for service...
1) Did any part of the heater get wet recently? With all the rain that much of the country has had, this could be a possibility. It's happened to me on occasion, especially during wind-driven heavy downpours even with top of heater being covered . Once it dried out, it fired right up
2) Check burner tubes for spider webs or mud dauber nests that could restrict gas flow to the burner.
The heater is outdoors, uncovered. It's been dry for over a week. I didn't turn it on when the pool was opened, but of course when I needed it it didn't work!
I did clean it. I also cleaned the various connectors, some of which tend to oxidize over the winter. No luck. Pressure switch was replaced two years ago, thermocouple a few years ago. Would faulty high limit switch cause this? It seems to be the only part that hasn't been replaced.