Someone moved my things out of my apartment for me while I was out of the country. They then kept my things at their place and since I’ve returned they have refused to return my things to me.
Can I get the police involved?
One option is to bring a civil lawsuit to obtain a court order for the return of your property, something that usually proceeds on an expedited schedule (often one to three weeks from filing to an order). The traditional name of such a lawsuit is "replevin" although the modern and minority terminology for this kind of lawsuit is civil action for "claim and delivery."
You could ask the police to intervene, and they might do so, but are not required to do so without a court order. This can take the form of a criminal theft charge, or could take the form of what is called a "civil assist" when the individual comes to the place where the property is held in the presence of law enforcement which assures a peaceful transfer.
Sometimes a court in another matter such as a divorce, or domestic violence criminal case, will enter an order authorizing or directing that a civil assist take place.
The appropriate remedy depends to some extent upon the reason given for not returning the property. One valid defense to not returning property is that a lease created a lien in the personal property left at the premises for unpaid rent, or a lien for moving and/or storage charges. The availability of such liens varies from jurisdiction to jurisdiction and based upon the precise details of the situation. Another valid defense would be that the person in possession of the property was not satisfied that they knew who owned it.