if it's a quill stem, first unscrew the bolt a little bit ,check if it's moving up and down, then if it's not (should be up) hammer it down, then lubricate and try to move the stem sideways, once that happens you're nearly there, if the problem perstsists, then put the frame upside down pour coca cola in there and let it soak overnight, that shold melt the rust and your problems.
good luck
if it's a quill stem, first unscrew the bolt a little bit ,check if it's moving up and down, then if it's not (should be up) hammer it down, then lubricate and try to move the stem sideways, once that happens you're nearly there, if the problem perstsists, then put the frame upside down pour coca cola in there and let it soak overnight, that shold melt the rust and your problems.
good luck