Did you test the fuse to ensure that it was good when you pulled it? Sometimes they don't 'look' blown when they actually are.
Is the water in the heater hot when trying to fire the heater on gas? If it's hot enough, that will keep the gas side from firing.
Have you checked the Hi-Limit/Eco switch and thermostat to see if it's passing power? There's one for both the AC and DC side. It's under a cover in the heater compartment near the burner assembly.
Here's a troubleshooting guide
if you don't already have one.