Undo the gland nut and lift up, spray loads of penetrating lubricant. eg wd40, plus gas or anything else you hand. Leave for a couple of hours, then try and turn the handle 1/8 of a turn back and forth. Then hopefully you can turn it a bit more in both directions, keep going until you can completely turn it off. Then open all the way and close 1/2 turn.
Andy
Andy