If you have a tankless water heater that doesn't heat, there's going to be only a few things wrong with it. If it was working before it probably has plenty of gas. So you can scratch that off the list. Every once in a while you're going to get sediment inside the tankless water heater and that needs to be rectified by adding a filter and flushing the heater out. The quickest and best thing to do is to call somebody that, perhaps, installed it for you or has installed it in the past and knows exactly what to do.