If your heater is like mine...
There is a small window about the size of a quarter inside
the panels on the front part of the heater.
The panels must be removed to see it.
It might look like a hole but glass is there.
Inside the window an L.E.D. light might be blinking.
If this is the case, count how many times its blinking.
the number of blinks represents the nature of the problem.
if it blinks 3 times its possibly either a small outflow fan isn't working or something is blocking the vent which prevents a flow detector from making contact.
The reason it shut down is because of the fear of carbon monoxide poisoning due to no ventilation.
Try looking into the vent pipe from the top of the roof to see
if there are any obstructions.
If there are no objects blocking the vent pipe then maybe a fan or transformer has malfunctioned.
The light blinking 3 times refers to the combustion air switch.
It works as a vacuum chamber to work the contacts inside it.