The clutch disc is likely rusted/stuck to the flywheel or the pressure plate. Pretty common on old stuff that's been left to sit up for a long time. Sometimes they get stuck pretty tight, too. In the past what I have had to do to un-stick them is start up in gear (usually reverse) and drive the vehicle into something solid like a tree with the clutch pedal pushed in. This will usually crack it loose. I usually do this in reverse to take advantage of the lower ratio and also use the frame horn or rear bumper to make the contact with to avoid damage to any sheet metal.
