Are you using F type transmission fluid ? If it was used for any length of time with Dexron the clutch plates will be shot and will need an overhaul. If it just sat for a long time you may be able to make it work by just riding it a lot and changing the transmission fluid often, if it still won't work you'll have to open it up.
Want to post in this forum? We'd love to have you join the discussion, but first: