1) Check the clutch cable to see if it is stuck. You can also play with the lever, on top of the transmission, which the cable attaches to, to make sure that rotates normally -- hook up a Vise-Grip Pliers to move it.
2) Check the oil to see if it is over-filled -- with gasoline! Drain, change, both oil and filter. Usually if the fuel petcock is left on/open, gasoline will drip onto the top of the transmission, seeking the lowest point... Which is the oil pan beneath the transmission on DOHC models; not sure if it will collect in the transmission, on your SOHC, or not, but be sure to check it.
Report back with what you find. Cheers!
1979 CB750K (sold, 2012, but not forgotten)
1983 Kawasaki 440 LTD Belt Drive (sold, 2011)
1993 Kawasaki Voyager XII