check the oil in the tranny, is thereany/clean? try adjusting the lever. how long has it been since u started it? it could be that the oil is cold and sticking. if thats the case, warm up the engine than try to shift into gear.
If those don't work you might be looking at burnt bearings/bushings...you can check by seeing if the clutch plates even disengage when you have the cover off. If they don't even separate, depending on what's going on...you could be looking at a total rebuild on the bottom end because possibly of the bearings/bushings on the primary shaft :/