Futzing with gas when you don't know what you're doing can be quite dangerous for both your health and your property! A magnet doesn't go 'bad', but it might be possible it got corroded. The thermocouple generates a small electical current when it is heated. If the wires are shorted or open, it can't control the valve. Basically, it triggers the gas valve so it can work when it senses the pilot is lit. Not sure what you adjusted, but it was probably coincidence things worked after that. It may have been a gas/air adjustment, but that wouldn't affect the burner's ability to fire, only on how well it actually burned once on. Other than that, I've pretty much exhausted my knowledge on the subject. There may be another interlock somewhere - the manual should show them all.