If you currently do not have heat here are a few things to check.

Power to the furnace?

Make sure your furnace is getting power.  Check the breaker in your electrical panel and reset if required.

Thermostat

Check to ensure the thermostat is set to ‘heat’.  You will have three settings to choose from, cool, heat, off.  Set the fan to ‘auto’.  Move the thermostat temperature high enough to trigger the furnace to start.   Some thermostats have batteries, ensure the batteries are functional.  Check your fuse/breaker panel.

Furnace Door

Check to ensure the door to your furnace is properly in place.  If someone recently changed a filter or bumped it out of place then your furnace will not operate.

Furnace Filter

Dirty furnace filters are the most common causes for furnace failure.  If the filter becomes ‘clogged’ then the airflow is limited which will cause your furnace to overheat.  If you suspect your filter is dirty, change the filter then press the ‘reset’ button (if your furnace has one) and see if it will start to work again.

Outside vents

Ottawa receives plenty of snow each year, that combined with our strong winds it can block your outside vents.  If the air-intake or the exhaust vent becomes blocked your furnace will no longer work.  Clear all the snow away from your vents and that just might solve your problem.

Power switch to furnace

You can have power in the rest of your house but if someone turned off the ‘switch’ to your furnace it will no longer work.  Look for the power source to your furnace, that switch could be in your ceiling, on the wall close to your furnace.  It looks like a regular light switch, if it was switched ‘off’ then your furnace will no longer work.