If a realtor showed you the house when it was on the market, and it was within the previous 6 month period, you should go through the realtor. Sometimes contracts between seller and realtor will specify a shorter period, but generally, if a realtor showed you the house, they will still get a commission if you buy it within a certain period of time after their contract ended.
If you were never shown this house by a realtor, and the house is no longer listed with a realtor, then you can contact the owner directly. If you can get a phone number, then place a call, otherwise, write a letter expressing your interest and ask them to call you. I would not advise knocking on their door except as a last resort. At this point, the ball is in their court. They may turn you down, refer you to a realtor, or meet with you personally.