Both of my YS-110's died about a year apart at 9-10 years. At that point they're generally unrepairable because Sea & Sea stops producing the PC board.
If the strobes fire, they're probably fine. Besides your setting and distance from subject, make sure your strobes are timed correctly for the preflash. Take a picture in the mirror. If your flash doesn't show, your camera setting or your strobe setting is wrong for the camera.