Try tapping gently on the stuck valve to see whether it wants to move (try to have the piston near BDC to give the valve space to move). Also try to see whether the valve turns in its guide, even if it doesn't want to move up/down, for more info on how badly stuck it is. Another possibility is that you have something (rust, nuts from squirrels, mouse nest materials - the possibilities are near endless) in one (or more) of the cylinders that is preventing that cylinder from reaching TDC. If it is loose stuff, you may be able to blow it out (piston to BDC for clearance to open the valves, open valves, compressed air in spark plug hole), you might hear nuts rattle even if they don't blow out. You may end up needing to remove the head to see what is going on. Head removal will certainly let you see whether rust has developed on the cylinder wall above the rings and how bad it might be and may help you free up the stuck valve by tapping (again, gently) both directions.
