When it gets cold and temperatures drop below 20 degrees, heating oil can wax or gel which will cause a furnace fuel line to "freeze up" or "clog". This of course will stop fuel from flowing. There are some steps you can take to prevent this from happening.
1. Use a Heating Oil Additive that prevents waxing and gelling. This is the easiest thing to do. You can purchase this additive yourself or from your oil dealer. Simply have the delivery driver pour the liquid additive in every time you fill your oil tank with fuel.
2. Wrap pipes with insulation that prevents moisture from getting in and tape closed with heat resistant tape.
3. Installing larger than the standard diameter (3/8") oil fuel lines will help keep oil flowing longer than a narrower line. You can have a 1/2" diameter oil line installed.
4. Some will switch to using kerosene instead of heating oil since it doesn't gel. Or at the very least have some kerosene added to the oil tank to mix in with the heating oil. This also lowers the flash point, but since kerosene is more expensive than heating oil, most skip this option.
So you haven't been pre-treating your fuel tank with a fuel additive. Your furnace has been working well and in good shape and then one day after a cold freezing night it's not lighting up!
You wonder what's going on? If your furnace has been working fine up to this point, then it could be that your fuel lines are frozen.
Before you call the technician to come out, try pouring a bottle of the fuel additive used to prevent gelling into your fuel tank, to de-ice your fuel line. It will in some cases free up the line, plus you will avoid a service call!