In Maryland, if the other side refuses to return property that is rightfully yours, you may go to the District Court and file a Writ of Replevin -- if the Court agrees with your position, it will grant the "writ" and the Sheriff may assist you in retrieving your belongings. Hopefully, you can resolve this more amicably. What I would NOT advise you to do is to take the law intro your own hands, trespass upon the towing company's property, and retrieve it yourself.
To get the form you may need, log on to http://www.kramerslaw.com/state-court-forms/maryland-court-forms. There, you will find Maryland forms that include the Writ of Replevin suit toward the very top of the list.