Could be a bunch of things like a frozen piston or something internal blocking the gears. I had the same situation and it turned out that the nut on the clutch adjuster came off and jammed in the gears. I could hear it ticking and then when I stopped in wouldn't let me kick it over. All I ended up doing was standing it on it's side, taking the clutch cover off, putting the nut back on and then putting everything back together.