When you pull the lever it causes a piston to push a rod that passes through the crankcase to the clutch, the rod also passes through the cover plate for the front sprocket and it can get very very dirty/greasy/gummed up in there, the rod may be stuck in the hole and not returning??
My first thought would be to strip the mechanism and make sure its clean and working properly. Its easy enough to do & there are tutorials on the VSRI site on the whole thing.
You haven't lost your clutch fluid?? (check the LH reservoir level)