This is a common problem. If you're sure you have enough tension on the clutch cable, there should be little to no slack in the engine end of the cable where it attaches to the clutch arm, then there is a good chance the clutch pads are stuck. The easiest fix is to push in as far as you can on the arm with hand pressure. It takes a lot so wrap the arm with a rag and put some force into it. Many times you'll actually hear the clutch free itself with a 'clunk' sound. If this fails, remove the clutch actuator cover and look for the 5/16" diameter pin which should be protruding from the center of the drive sprocket. Using a plastic headed hammer, or a block of wood to protect the pin, give it a couple of good solid raps with the hammer. If these methods don't help you get back to us. We'll get you going.
Tom