If you were registered to vote before you left, check your state’s website to see if your voter registration information is still available; it may include your voting address.
Other documents–such as old letters, address books, birth certificates, or tax returns–may have your old address. Relatives or friends may remember your old address. If you know the general location, you can use Google Earth to look for the street address. Finally, you can contact your Local Election Official (contact information here)–they typically know the area well and may be able to help you if you provide a description.