I suppose the obvious questions are:
How does it work - is it running the absorption refrigeration gubbins with an electric heater on 12V/mains, or using a separate peltier or something? If separate, it could have lost refrigerant...
Is it actually burning gas? Check for water vapour at the flue, or something. If not, I'd suspect a thermocouple interlock failing safe, rather than a physical blockage, though I suppose a valve could simply be stuck.