If the clutch hasn't been actuated in a while, it's possible all the plates are stuck together. A wet clutch system uses the oil to introduce a bit of slip, and when the engine doesn't run, there's no oil between the plates, so they adhere a bit more than usual.
You can help ease that tension by putting the trans in 2nd or 3rd gear, pull the clutch in, and rotate the rear wheel. It'll be pretty stiff at first, but it should ease up. That happened to my Shadow last winter.
Let us know if that fixes the problem. Keep in touch.
Luke
Used to have a 1979 CB750L, sold it as a parts bike, now riding a slightly modified 1984 VT700C. Network/Field Engineer. Central OH, USA, Earth, Sol System, Milky Way Galaxy, Universe.