You will have to sue them. There is an action called "replevin" which would ask the court to order your ex-employer to turn your property over to you. You would have to prove that you own the property; that you have a superior right to possession of the property, and that the employer doesn't have claims against you that give it the right to hold the property. It can be a fairly technical type of claim, and will probably cost you more than $5000 in attorneys' fees.
Alternately, you can sue for the value of the property they are holding. This gives you money instead of the actual property. The employer could still defend, for example by claiming that you owe them money for something.
You have several options available to you depending on whether you want the property itself returned or are willing to accept a sum of money you feel is fair to compensate you for the loss of said property. A replevin action against your former employer could prove to be more complicated than an action for monetary damages. The course of action you take is really a matter of personal choice but in either event would require the assistance of competent legal counsel as the employer appears likely to stage a defense on some basis. Feel free to contact my office if you wish to discuss the matter in greater specifics.