Is the mast stepped or off the boat? Is the sheeve simply seized or is it dammaged from corrosion or abuse?
I'd first try a good penetrating oil such as "PB BLASTER". It's an aerosol spray and the best thing I've ever found for freeing frozen fittings on my boat.
Frozen masthead fittings can sometimes turn ugly. If you can free everything up - I suggest you disassemble the everything and apply a thin coat of synthetic lubricant such as "TEFGEL" to the SS shaft before inserting into aluminum sheeves.
If worse comes to worst - SELDEN carries a great selection of mast hardware at reasonable prices.