Be sure the clutch cable is adjusted correctly, and moving freely. 3/8-3/4" free play at the end of the handlebar clutch lever. Also be sure that it is operating the clutch actuating arm on the engine housing.
If it still won't release, try krytron's advice of pushing it back and forth with the clutch lever pulled to try to break it loose.
If that doesn't work, I would try to ride it. You may have to push start it in 2nd gear to get going. Then go through some acceleration/deceleration with engine braking cycles with the clutch lever pulled all the way into the handlebar. It may come free when it warms up some and the thermal expansion moves things slightly.
You can up shift to higher gears without the clutch by matching rpms. Higher gears should put more torque on the clutch from the acceleration/deceleration cycles. You can also downshift without the clutch if you match the rpms.
Just be careful that you do this in an area with no traffic and plenty of room to maneuver so you don't get into any tight spots, since you may have to stop the bike by shutting it off if the clutch does not come free.
If that doesn't do it, you will have to pull the right hand engine cover, and see what you have going on in there.
1999 Honda Rebel CMX250C
2003 Harley Sportster 883 Hugger
1998 Harley FXDWG Dyna Wide Glide
ALL IN! GO TIGERS!