I hope I am too late to fix your problem and you are warm again. I had this happen to me two or three times. What happens is at the bottom of your oil tank all the nasty sludgy oil has asccumulated. This gets sucked into the burner and clogs up the small openings in the nozzle. Typically you need a furnace repair person to come out and clean the nozzle. It is also a good idea to change the oil filter at this point because is is probably clogged as well.
The best long term solution I have found to your problem is to contract with an oil company to have the oil on automatic delivery.