First get a can of silicone (not petroleum based)spray lubricant and liberally spray ball fittins on either side of the valve where the handle attaches. Go ahead and make a mess....you can clean it up later. Do the above 2-3 days in a row. Then get a 12" piece of PVC pipe (not metal and don't use a hammer) to use as a cheater and gently make the valve handle move some....settle for 1/8". Spray again with silicone and move the handle a bit more. The idea is to gradually free the valve, not to do it all at once.
You can also remove the drain plug in the bade of the valve and replace it with a grease fitting and apply silicone grease to the grease fitting with a grease gun then try to move the handle with the short piece of pipe.
Good luck with it.........
