This happens to mine a lot, I usually tap it with a block of wood and a mallet to free it up. If it's bad I have someone put some pressure on the wheel in the same direction as I'm tapping the ram.
Check the grease and make sure it isn't all rusty, if so then re grease it. I store my boat with the ram retracted now so that it isn't exposed to the elements, seems to freeze up less that way.
If it comes to a point were you can't get it loose and need to use some wd40, make sure you dry it real good and re grease it real well. The wd40 will wash away the grease and make it seize up worse.