Dumb question. Are the batteries in correcty? Ceck polarity. If they are in ok, check to be sure the wire connecting the external contact to the terminal inside your battery case is not broken. Best way to check it to put a volt meter across the outside contacts with the batteries installed. You should get abut 10 to 12 volts. In a perfect world 12 volts. If no voltage, the problem is with your batteries or the connection points inside the battery case.
If you do have voltage, check the springy contacts in the bottom of the detector housing, where the battery slides in. If oneis bent, get a stiff wire, make a hook on the end and GENTLY bend the contact back in positon.
If not the contacts, the problemis probably in the control head of your detector.
If it is, you will have to send it off for repair.
Let us know what you find out.
Ray S