Water Heater Won't Start

It's old, but if it's a simple fix, that'd be great.
A couple months ago, it wasn't as hot as it should've been, but it came around and has been okay until today. The pilot is lit, and the exhaust fan runs (until I unplug it), but the burner won't light. I've lightly tapped the gas valve. Nothing. (It has an exhaust fan because the exhaust stack is horizontal.)
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