faulty clutch,won't disengage,cable not broken

I own a 1974 Bonneville 650 which hasn’t been ridden since last year. I have kept it clean’ stored in a shed in my yard.  Anyway I took it out the other day and started it, but when I put it in gear  it conked out because it was in gear, with the clutch  handle pulled in…….I  opened the little slotted cover on the gearbox and could see the cable wasn’t broken.I appreciate any advice