As the last two posters have said, the thermocouple seems to have gone bad. Even cleaning it won’t work. It’s a relatively inexpensive part. I’ve replaced mine on my water heater and also my furnace. Pretty simple repair to do on your own.

What I did is searched Amazon for water heater thermocouple and found one that matched what I had. There are a couple different types. Try including the water brand and model in your search. Such as “Rheem MODEL water heater thermocouple”