Does the handle screw on or is it held on with a screw through the middle of the handle into the stem? If it's the latter then I'd just put a screw in the hole.
When the valve leaks by, i.e. not stopping the flow through the radiator, then the washers and/or seats are bad. When the valve leaks out to air (mess) then it's the stem packing that's bad. There's usually a cap with a hole in it for the stem that screws on the valve like a nut and holds the stem in the valve. The stem packing goes on the stem before the cap/nut goes on.
So if the water is coming out the center of the stem put in a screw. If the water is coming out around the stem then try tightening the packing nut (that cap/nut thingy). If that doesn't work you'll have to remove the packing nut and wrap some stem packing (teflon tape works) around the stem and replace the nut. That can be pretty messy with pressure on the system, especially if you don't have experience.
BTW, any valve in the house that doesn't get moved for months (years?) at a time, it's a real good practice to loosen the packing nut before moving the valve then tighten it up again. It keeps the packing material from getting torn up and keeps valves leak free longer. Of course when the water level is between your upper lip and nostrils, I wouldn't bother with it.
Sorry it took so long to get back to you. Mom pulled rank.
