This isn't the first time. I'm an old hand at it now.
These valves, though expensive, go bad after a few years in that suddenly they refuse to turn on (It is fortunate that they fail in this manner - what if they wouldn't turn off?).
The repair is simple, but the flange nut on the end of the valve (the pipe that goes to the burner) is difficult to get to with an open end wrench). One can take loose the supply manifold (the pipe that all the other valves are connected to) and that helps a little bit.
I now have three spares (one for each type of burner (low BTU, high BTU, and griddle-in-the-middle), because another one will fail soon.