We have a Weber Genesis Gold gas grill (purchased new at
an auction in support of our church). The first time I used it,
it was difficult to light, but eventually started and worked fine.
The second time I used it (a couple of days later) it was
easy to light. The next time, it wouldn't light at all. When it
wouldn't light using the auto start feature, I tried to light
it manually and that was a no-go, too. It appears that no
gas is flowing from the tank to the burners. This same thing
happened last night.

Does anybody have ideas on what might be happening?

Thanks in advance.