Well mclain was working now it's not, help

My well mclain boiler was working today until i hit the thermostat to cool while working around the house, when i relized that it was getting cold i put it back to heat and my burner is not turning back on, i had the same problem about a month ago and it just started to work. tonight i checked the low water sensor power and made sure all wires were connected but still nothing,

Got a model number, fuel type, and better description of the system (age, history, etc.)? There's a good handful of reasons why a burner won't fire, but not all are relevant to all makes & models.