The rules vary from state to state. Additionally, there are usually some offenses that cannot be expunged. Also, usually, you can have absolutely nothing else of a criminal offense on your record; this includes misdemeanor offenses, even misdemeanors that are violations of the motor vehicle code like drunk driving. Further, the time frame is usually a certain number of years from the date of completion of the sentence, not the date of conviction. For example in Michigan, (where I work), if you were convicted six years ago and placed on four years probation you would not be eligible for expungement because five or more years had not elapsed from the completion of your sentence; discharge from probation. With the information that you have provided you should contact a local attorney who has experience in criminal law with an emphasis on post-conviction/appellate work.
If it is a conviction, then the only remedy is to apply for an executive pardon through the Board of Pardons which is located in Harrisburg. With a skilled attorney, the statistical likelihood of obtaining an executive pardon is high.